Yeah, the plugin overhead varies massively. An SSL channel strip is going to be heavy, of course. The 610-B tube preamp/EQ is also heavy, to name one off the top of my head. On the other hand, many of the plugins (e.g. the 1176LN legacy) are quite light and sound good to me. Also bear in mind you’d only be applying them to a stereo buss, it’s not like trying to put a channel strip on every one of 10+ inputs (I have 8 channels of ADAT coming into my Apollo so I’m balancing CPU across 10 channels quite often). What would you be thinking of using? Buss comp? EQ?
In any case, a “normal” interface won’t do any of that stuff and you can always just treat it like that, bring it through dry and add your existing fx/EQ in the DAW (and not get sucked into buying more and more plugins). But then I suppose the question is, are you getting the value out of it.
FWIW I love my Apollo. I haven’t properly A/Bd it against the Focusrite Saffire I was using, and I am aware confirmation bias is very much a thing, but I swear the monitor output sounds way better.