I use two timers, one for real time and one for in game time. Every time the in game timer updates (every five minutes), it causes a really big lag spike for up to like 10 seconds. Wondering if anyone has had this issue and knows how to fix it?

It's called 5 minutes lag, because your game autosaves every 5 minutes (not related to your timer) there's no way to get rid of it in solo runs but the time that you lose during the lag spike isn't taken into account by the in game timer anyway, so you're good

I don't know if I should ask a different question on the same thread, but is there a way to disable the autosave it for co-op?

@Seanomunyango yes there is. the command is /save-off