That's really strange behaviour. Is that specific for that scene or you can reproduce it with any new scene too?
I would suggest you to check your scene for the "Retimers issue" as there is a known 3ds Max's bug, which is affecting XRef objects (because Forest Pack is using XRefs it's slowing it down). Feel free to you to use the Cleaner
script did by Andreas Meissner, also the Clean Retimers
script works very well. If that's the case, on Autodesk's Community
you can find more about this 3ds Max's bug and how to remove it using MAXSript (apart of scripts listed above).
To double check that's not a licensing service related issue (it shouldn't be), you can change temporarily the license mode on your computer to Stand-alone
mode, using "Start Menu->Itoo Software->Forest Pack Pro->Change License Mode
" (you'll get error, but no "license search" will be performed and this way you can compare if there is any difference).
Anyway, please feel free to send us
your scene to check it here. Thanks in advance.