I recently bought and installed a 4200nex which replaced my older avh-p4400bt. I bought the 4200 mainly because I liked my 4400 and I'd read many good reviews, but I am severely disappointed. This new unit is very slow and lags with any touch input. When I press the home button I can count to 3 or 4 before it switches. Volume is the same, I'll try to turn the volume up, nothing happens, I press volume up more and then it will suddenly shoot up. Carplay is also very slow, I'll press an app and then wait for it to open. This is with a 7+ so I know it's not my phone slowing things down.

I'm about ready to send it back, this is not at all what I thought it would be. My old pioneer was much faster than this so I don't think I'm being unrealistic as the 4400 was at least 4 years old.

Thoughts on this? Defective unit or just unreal expectations?

Have you tried pulling, let it sit a minute and then plug it back in. If that doesn't work you most likely have a faulty unit, the nex line is very responsive, nearly Instant

You have a defective unit, it shouldn't be that slow.  Mine's responsive and fine and volume button presses are always instantantaneous.  Also, once in carplay, the speed is actually dictated by the generation of iphone, because the software actually runs from the phones processor.  But volume changes have nothing to do with the phone, which leads me to believe your unit is defective.