Upvotes are clicked a lot but I guess they are just an api call and can be tuned to be high performance.

I rarely click anything on stack overflow and neither do most people I know. The only ones I know who do that are stack overflow evangelists. I'd guess very few people have an account and use it.

I've taken to using upvotes for bookmarking, that way when I inevitably look the same question up again six months from now I'll know which of the answers I thought was most useful last time.

I’m in between. I have an SO score of just 1000, but I vote a lot. Mainly because I’m logged in via oauth anyway so it’s no hassle to do so.

Upvotes are actually ideal for an in-memory service like Redis that fsyncs less frequently. Even if you lose the last few reads (highly unlikely, with some care), it's not the end of the world.

it's also not super critical to show correct numbers in real time for that so there's that.