How CSS Properties Affect Website Performance

Modern web development constantly balances visual polish with performance. I’m writing this because many teams only notice CSS performance issues after users report jank, scroll lag, or poor Core Web Vitals, even when the UI “looks fine.” CSS can create beautiful interfaces, but certain properties increase rendering cost and system load in ways that aren’t always obvious during development.

Many sites slow down due to effects like blur filters, complex shadows, and inefficient animations. This guide breaks down which CSS properties typically impact performance most, why they cause slowdowns, and how to optimize them without sacrificing design quality.

CSS Performance Lessons from Building a Video Platform

Recently, while working on a video streaming interface, I encountered an intriguing performance issue. The application featured video previews with blurred backgrounds generated from thumbnails. While the initial user experience was smooth, a noticeable degradation occurred as users interacted with more videos.

The Investigation

Initial debugging focused on isolating the change that increased rendering cost:

• Analyzing Swiper configurations
• Reviewing recent code changes
• Monitoring system resource usage

The key signal came from GPU utilization:

• Without blur effects: GPU usage stayed around 42–70

• With blur effects: GPU usage spiked to around 130

The performance change correlated directly with blur rendering cost. Blur intensity (radius) increases per-pixel computation, which can push GPU workload sharply upward when applied to multiple elements or during frequent updates.

This dramatic difference in GPU utilization was solely attributed to the blur effect implementation. The internal computations required for the blur effect, based on the provided radius, heavily utilized the GPU for calculations.

High-Impact CSS Properties That Can Slow Down Your Website

Resource-Intensive Properties

Some CSS properties are consistently expensive because they increase paint/compositing work and per-frame computation:

Filter effects

• blur() is demanding due to per-pixel processing
• Larger radii increase computation cost significantly
• Multiple filters amplify the impact

Complex shadows

• box-shadow with large blur values
• Multiple layered shadows
• Shadows recalculated during interactions

Advanced operations

• Transform combinations
• Complex animations
• Filter chains

Impact on System Resources

These properties impact performance through measurable resource pressure:

GPU/CPU impact

• Higher processor utilization
• Thermal throttling risk on sustained workloads
• Battery drain on mobile devices

Memory management

• Higher RAM usage
• Cached computation overhead

Performance metrics

• Reduced frame rates
• Slower time to interactive
• Increased input latency during scrolling and interaction

4 Common CSS Performance Mistakes to Avoid

1. Filter Overuse

A common mistake is applying heavy filters like blur() across multiple elements or inside frequently-updated UI states. When filters are applied repeatedly or at scale, rendering cost multiplies quickly and becomes visible as lag during scroll and interaction. Our case study showed how this seemingly innocent choice can lead to significant performance degradation.

2. Shadow Complexity

Complex shadows become expensive when blur values are high, when multiple shadows stack, or when shadows change during scroll/hover. The cost becomes more noticeable on lower-end devices and on long pages with many cards. This becomes particularly noticeable during scrolling or hover interactions.

3. Inefficient Animations

Animating layout-affecting properties like width, height, top, or left increases layout and paint work. Prefer compositor-friendly animation properties (typically transform and opacity) to keep motion smooth and reduce main-thread load.

4. Resource-Heavy Images

Oversized images increase network cost and can add ongoing rendering overhead when resizing and repainting occurs. Use correctly sized assets, modern formats, and predictable dimensions to keep both loading and rendering efficient.

CSS Optimization Strategies

1. Hardware Acceleration Optimization

• Use transform and opacity for animations where possible, since these are typically composited more efficiently than layout-triggering changes. Apply will-change only to elements that truly need it, and remove it when the animation ends to avoid unnecessary memory pressure.
• Apply will-change property only on elements that actually change to help browsers optimize rendering without wasting memory resources.
• Choose compositor-only properties for animations to offload work to the GPU and reduce main thread processing time.

2. Smart Filter Usage

• Keep blur radii modest (often under ~20px) because rendering cost rises quickly with larger values. Avoid applying filters inside loops or to large containers. Where possible, replace dynamic filters with static alternatives (pre-rendered assets, gradients, or cached effects) to prevent runtime performance penalties.
• Batch filter operations instead of applying them in loops to prevent repeated calculations and minimize browser rendering cycles.
• Replace dynamic filters with static images or CSS gradients for decorative elements that don't require real-time updates.
• Generate and cache filter effects during build time for static content to avoid runtime performance costs on user devices.

3. Image Management

• Set correct image dimensions in HTML and CSS to prevent browser resize calculations and reduce memory usage during page rendering.
• Set explicit image dimensions to avoid repeated resizing work. Use modern formats like WebP/AVIF where supported, lazy-load below-the-fold images, and apply compression that preserves visual quality without bloating payload size.
• Implement lazy loading for images below the fold to reduce initial page load time and decrease memory consumption during page load.
• Use appropriate compression levels and remove metadata from images to balance visual quality with file size for optimal loading performance.

4. Performance Monitoring

• Monitor GPU/CPU usage in browser dev tools to identify spikes tied to CSS effects. Track frame rate during scroll and animation, and test across a range of devices to validate that improvements hold under real-world hardware constraints.
• Track frame rates during animations and scrolling to maintain 60fps, detecting CSS-related bottlenecks that cause stuttering or visual lag.
• Test website performance across different devices and browsers to ensure CSS optimizations work effectively on varying hardware capabilities.

Best Practices for CSS Performance

1. Selective Enhancement
   • Apply heavy effects like blur and large shadows only where they meaningfully improve UX. Build a solid baseline experience first, then progressively enhance for devices that can handle complex effects. Prefer simpler transitions when possible, reduce animation intensity on mobile, and remove effects when elements are not in view to reduce ongoing resource usage.
   • Build a solid base experience first, then progressively add complex animations and effects for devices that can handle them properly.
2. Smart Animation Choices
   • Limit the frequency of performance-heavy animations using debounce techniques to prevent excessive calculations during rapid events.
   • Choose simple CSS transitions over complex keyframe animations when possible, as they require less processing power and are easier to optimize.
   • Scale down animation complexity on mobile devices to prevent battery drain and maintain smooth performance on less powerful hardware.
3. Resource Management
   • Remove or disable CSS effects when they're not in view, and clean up unnecessary styles to prevent memory leaks during long sessions.
   • Regularly test memory consumption across Chrome, Firefox, and Safari as each handles CSS effects differently, especially for long-running animations.
   • Use real devices across different performance tiers to ensure your CSS choices work well for all users, not just high-end devices.

FAQs

1. Which CSS properties hurt performance the most?

Filters (especially blur()), large/stacked box-shadow, and animations that trigger layout (like width/height) typically have the biggest impact.

2. Why is filter: blur() so expensive?

Blur requires per-pixel computation. Higher blur radii increase the amount of work needed, especially when applied to large areas or many elements.

3. What CSS properties are best for smooth animations?

transform and opacity are generally the safest choices because they can often be composited more efficiently than layout-triggering properties.

4. When should will-change be used?

Use will-change only for elements that will actually animate, and remove it when the animation ends to avoid unnecessary memory and rendering overhead.

5. How can I tell if CSS is causing scroll lag or jank?

Use browser performance tools to check frame rate, GPU/CPU spikes, and long paint/composite times. Compare behavior with and without effects like filters and heavy shadows.

6. Do shadows always slow down a website?

Not always. Small, simple shadows are usually fine. Performance issues appear when shadows have large blur radii, stack in layers, or animate frequently.

Our Final Words

Creating visually polished websites without sacrificing performance requires intentional CSS choices and measured trade-offs. By understanding which properties increase rendering cost and applying targeted optimization strategies, teams can maintain design quality while improving responsiveness and Core Web Vitals.

Remember to:

• Test performance across devices and browsers
• Monitor resource utilization, especially on mobile
• Balance effects with performance budgets
• Use progressive enhancement for complex visuals

The goal is consistent: keep experiences smooth, fast, and reliable while still delivering modern UI polish.