Not every M4L plugin will work on push. Despite what has been said here a few times. Controls need to be made available on push by the m4L device’s developer for starters. Ableton has said that M4L will become more integrated down the road but for now only a select few devices work fully and have all controls available. If you’re unfamiliar with M4L and its capabilities I’d suggest focusing on the stock plug-ins first as that is already a huge amount of stuff that is available and powerful. There’s a dedicated topic in this forum where people list M4L devices that currently work.
Regarding UI sluggishness, you must be adding a lot of heavy audio effects racks or complex instrument racks for that to happen?. 
. Haven’t encountered any of that so far and I’m running push 3 at a buffer of 32 currently. Feels responsive as any device should. But again, I’m not really stress testing it nor am I trying a lot of diffeeent M4L devices yet as most simply don’t work fully yet :).
Maybe start a clean project and just start creating your own effect chains with intention :).