Help with very small hold time violations

An invalid hold requirement can cause the fitter to add delays to meet that requirement, which can cause congestion. (Assuming Optimize Hold timing is set to all paths, which is what tells the fitter to add routing delays to meet hold requirements.) If it's small, it's probably not a big deal. Where it can be a problem is if a user adds a multicycle -setup of 2, or something like that, and forgets to add a multicycle -hold of 1, whereby they have a hold requirement that is a whole clock period, and hence the router needs to add a lot of delay to try and meet this.