I've seen the warning while trying to increase timeout saying that it could effect overall performance. How exactly this work? Will it put more load on CPU or just work slower in general?
I am trying to find solution for FF errors even when file is uploaded. Increasing buffer didn't work. I figured that FF maybe needs more time to return the link so I tried to increase timeout.

Not sure if this actually solved the problem. On small sample with timeout set to 60s I had 0 errors and 0 double uploads.
Edit:
Problem persists with larger files, double upload and no link.

Timeout has nothing to do with performance.

If you see error "Read timeout" - yes, increase timeout and it will help. That's the only case you should increase the timeout option.

If you see some other error and it should not happen - PM me more details and I will take a look and fix it if needed.