@ Bormax - tried your scene and I can confirm this strange behavior, did some quick\erratic\chaotic testing on it using corona's 1.4 rc1 and it seems that the problem is material related - tried replacing those with a default corona grayscale material and bam - scene parsing takes no more that 10-20 seconds.
One thing comes to my mind - could it be that you created that scene using some previous version of forestpack and then upgraded to the latest one ? Why that could be important, well I presume Itoo could change those preset's (and any others too) models as well as change\tweak some of the maps used by those presets but the map and preset naming remained the same thus your grass materials is using old materials and expect to use old maps for them as well but those just aren't the same anymore and as a result are producing those long parsing times etc., well it's just a guess but materials were changed for sure and even the way maps are loaded in recent versions was changed as those are imported as corona bitmap now.
Still, even if that can explain why it parses the scene as it should when new presets are used, it does not explain why it did not occur with older corona's daily builds and not in the least explain why new library preset cannot be used in that same forest pack object in your scene - perhaps you should ask that in itoo's forum for I don't think that's corona related at all... Nor does that explain why such things do not happen to my scenes whether forestpack built-in or custom presets were used - I even tried rendering some of my older forest pack heavy scenes
http://c2n.me/3xCunUK and there the scene parsing time there is completely alright, well, given the quantity of heavy objects and the whole scene scale as that fragment is just a tiny part (thought most detailed one, with cars, photogrammetry made and railclone distributed stone walls etc) of this huge scene
https://www.flickr.com/photos/119850875@N05/18340080264/ As a side note, I noticed your scene is using generic display units which are probably not the best choice when using such very much scale dependent plugins as forestpack, still the system units are in cm in that file so perhaps that's is not effecting anything. And as another side note - I'm not using the latest release version of FP but using the 5.0.2b beta build as my subscription has expired and that was the latest version available for it back when - I'm not extending the subscription mainly because everything works alright as it is +I'm waiting for them to add 3ds max 2017 (as it is awesome and FP is one of the things stopping me from using it fulltime) and complete corona 1.4 support as well ))