Hi guys,
just a small update directly from iToo Software which I received a while ago:
"Hi Dionysios,
As we can see from some further testing, the Forest object itself is built "almost instantly" (IForestRenderBegin > IForestRenderEnd) as you can double-check by setting General - Log Level to Debug in any Forest object in your scene, before sending it to the renderer (optionally by setting forceLog=1 in the forestpack.ini)? I'm afraid we can't do much more in this case at the moment (like debugging the rendering process) and I hope you/we'll get some more info regarding it from the "Corona team" soon. If you'll be interested, the mentioned log file is located at:
C:\Users\<user name>\AppData\Local\Autodesk\3dsmax\<max version>\ENU\Network\Max.log
Once again, big thanks for your collaboration, your feedback is very important to us."
In a few words, the Forest plugin seems to do it's work correctly. The long parsing times should be a problem of the rendering engine at this point.
I guess you know it but I thought was useful to paste here their answer.
Thanks!