That's strange. Are you using MIRx Mode already? Or in other words: Are you using multiple Room EQs? Or is it just an "old" project that gives you trouble?
The reason I'm asking this question is that this is the only aspect that has changed massively processing-wise. Everthing else should be more or lesst identical to the previously released version.
What do MIR Pro's Preferences look like? Did you change the MIR engine's own latency?
Did you change anything in you ASIO-settings?
Is Cubase running at the same time?
/Dietz - Vienna Symphonic Library