The time MIR Pro needs to re-calculate all impulse response data is only depending on the number of Icons on a Venue's stage, and on the number of chosen output channels (surround will take longer than stereo). An active Secondary Microphone will need additional computing time.
The number of loaded samples is only a factor when you're actually loading a new VI-Frame. In case of Vienna Instruments Pro, you can speed up loading times by setting VE Pro's Prefereces to "Load all cells in disabled state" (in the "Instances" tab). Now you can have Vienna Instruments Pro set to "Enable cells on MIDI activity" - like that only those samples will get loaded which are really used by the arrangement you're working on.
BTW: It's a good idea to have MIR Pro's RoomPacks installed on SSDs to speed up loading times.
.... this might be a great occasion to announce that the next update of MIR Pro has been optimized _considerably_ in that respect, reducing the positional IR-prerendering calculation times to a mere minimum (especially re-calculation times after switching the Secondary Microphone off and on again). :-)
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HTH,