I don't understand why would there be a time slot in the first place... do people not own their own laundry machines in Switzerland?

People who own houses and apartments usually do but rented places often have shared laundries. In newer buildings every apartment has its own laundry.

It's pretty common to have a shared machine in the basement. If you're lucky, it's on the -2 sublevel somewhere in a nuke shelter (yes, literally) and nobody cares whether you use it at 3 am on a Sunday.

I think it also depends where you live in Switzerland. From foreigners complaining about it I deduct that Zurich tends to be relatively strict. Where I live (Espace Mittelland) this is a non-problem.

People in small apartments usually don't, people in houses always do - this is the same in the US though.