Found quality of RTT mirrors and monitors vary widely depending on particular vehicle.

Setting PiP quality to "ultra" makes most mirrors and screens run at a much nicer framerate. :)

Also, when in the gunner position of a vehicle, it's best to switch to the "optics view", which is bound to NUMPAD-ZERO or V, I believe.

I doubt BI will be able to improve the PiP framerate without drastically affecting performance, but there is always hope.

According to the scripts the PiP screens (including mirrors, monitors and LiveFeed) are fixed at 30FPS, or actually 0.3 frames per tick.

However if your graphical settings are too high, or your hardware not up to par, you won't hit the 0.3 f/t and it will slow down badly; resulting in extreme low FPS and laggy images.

It does seem that the PiP scripting is good enough to not slow down your overall FPS (just a little), so it might need some tweaking to fix this issue for specific hardware configurations.
For example; I have no problems at all with PiP, even with a medium setup.

I'm really loving the PIP as it stands now but I think it still needs some work, especially the gunner seat PIP. Although, as MadDogX pointed out I don't think that it can ever be made perfect without causing some major slowdowns in overall performance. Upvoted anyway :)

This is seriously becoming annoying. Spawned myself in a minigun boat (with the 2 RTT screens) and at 50 (probably more) fps the monitors were STILL a bit choppy, experimented with the quality settings and nothing.

By comparison the civilian pickup had two RTT screens, both in full color and high resolution and ran smoother? I really don't understand this, and the render screen doesn't even have shadows, (which I am still a bit sore over consdiering vbs2 does http://www.youtube.com/watch?v=TRraq0UdWU4 ) why is it not smooth? This isn't about frame rates anymore it's something else.

The hunter by comparison with its 3 pip screens in one scene at the same rate is as smooth as the actual screen itself though I suspect that has to do with the quality.

Yep, that is the case and it has nothing to do with the game performance settings. I was performing a series of tests lately with vehicles using custom made PIP active camouflage system and notice same issue. FPS of the surrounding environment was superb but the "camouflaged" vehicles fps was so drastic the effect was completely ruined. In side the vehicle cockpits screens using PIP had much better fps though.

I don't understand why mirrors need to be PiP. Various older games already have perfectly working mirrors (Il-2 1946 for example) without this incredible FPS lag.

This has been fixed.

At least with PiP on ultra, the mirrors/monitors now render nice and smoothly.