Just tried if RC3 is better with this, unfortunately it's not. 55 seconds of temporary freeze to access/display a Composite map. To be fair, it's not a simple one but I feel like this has worked way faster before.
I'll make some tests and try to find out if it's something specific about my materials.
EDIT
There's a 5 seconds lag in ALL material actions, even with a very simple material used as global override. But from there the lag is growing with increasing complexity which I guess is natural. Still, way less responsive than it used to be for me.
BUG ALERT
I found a bug with the CoronaMultiMap not being rendered consistently, again. With IR, It'll randomly change colors when accessing the material and modifying maps or material values for example. It may stay the same when rendered in normal mode, but it also may change, sometimes... Really unpredictable, I can't find a pattern here but it's quite unstable.