Browser rendering engines are very slow at rendering large box shadows. I had a situation where a complex layout with fixed elements and large shadows slowed Firefox down to 2 frames/second for scrolling and DOM manipulation.

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Some advice:

• Be aware that by introducing fixed elements (e.g. sticky navigation bars) and large animations, you might force the browser to redraw large portions of the site when scrolling. When your fixed elements have shadows, this increases the screen area that needs to be redrawn, which might again require other elements to be redrawn, and so on.
• Consider using filter: drop-shadow() Show archive.org snapshot instead, which renders with hardware acceleration.

Also see

• Box-shadow v/s Drop-shadow Show archive.org snapshot
• Many box shadows will make your app unusable on smartphones and tablets
• Designing Beautiful Shadows in CSS Show archive.org snapshot