There’s a few reasons huds can cause fps drops or lag spikes. Honestly they probably have more affect on spikes than they do on general fps.
Main one is that if they make a hit indicator the intuitive way, having an animation play with the damageplayer event, then it can have a bunch of animation stacked on top of eachother if you play something like pyro, engi, heavy, which is the most notable “gotcha” for hud fps stability. That’s why most huds trigger an event to kill then run a custom hitindicator, triggered by the damageplayer event.
Uber trackers using the text-based version can also cause lag spikes when they trigger, which if a hud uses one and the user is unaware of it they might think it’s a random lag spike.
Another thing is the friendslist on the main menu, for some reason some people will get fps instability that can be fixed by removing the friendslist from the main menu. I’ve never experienced this but I have a theory it’s related to how many friends you have on your friendslist, more friends=bigger spikes, is my theory.