I have just stumbled upon a problem, that might be related to this.
Please take a look at following files. There is a simple scene with Engine COMP in it. Engine is outputting spinning banana and it works just fine until I get into exclusive mode (just by hitting ctrl+b shortcut in perform mode) and start moving mouse over lister. At this point both lister and engine output is lagging (though engine’s lagging seems to be much worse when compared to lag on lister).
engine_lag.zip (3.0 MB)
EDIT: @bangnoise please may I ask whether you think something strange might be going on with this example in terms of Engine COMP?
I am currently working on a larger project where I have observed somewhat similar lagging, but it is much harder to reproduce (it isn’t happening in the same situation as here - exclusive mode or mouse input doesn’t trigger it there). Nevertheless I feel like it might be somehow related (visually the lagging looks just the same, but once it starts, it doesn’t seem to stop so easily 
). I am kind of stuck here as it happened couple of times and I haven’t figured out what might be causing it. I was hoping that if you find some strangeness here, it might also solve the problem there. But maybe it is something completely different. Thanks