[meerita]

It matters.

First, not all users need your huge CSS file, in terms of downloading. Specially when they're on mobile phones and cache is crap.

A ~9000 rules CSS file will load and become COSSM slower than one that has 1000 or 10. It is linear increment. Make a test to see it for yourself.

A bloated CSS is more network time. Therefore, slower loading.

A bloated CSS will contain slower selectors, therefore slower rendering time and painting speed.

When you count all these things, you will see radical speed performance.

[BurningFrog]

Maybe. You need to measure these things and get a feel for the actual performance impact to make informed decisions.

Rule 1 of performance: Your intuitions are not reliable.

[saidajigumi]

Upthread, this commenter did post specific measurements from production projects.