there will simply be a button "Subscribe" or "Cancel subscription" next to "Reply" or "Fast Reply" and the like. There is no auto subscribe-function, and about the lag: let this be my problem.

@poison_us - the thread diverged quite a bit. There was my initial suggestion for an 'auto-subscribe' to threads in which you have posted. Then Devourer suggested a simple subscribe system for 5 (possibly 10) threads, which once subscribed to, you will receive a PM/notification which direct-links you to that thread when there have been new developments.

I disabled the old subscription system, because it would do x number of pretty resource intense queries on every single page refresh, where x was pretty much unlimited. I believe devourer subscribed to 60+ topics.
Devourer, before you do anything, I want a detailed plan on what you intend to do.
I may also start thinking of the best way to implement something like this.