Use Windows PerfMon on both the 2008 and 2012 servers, using the same set of counters. That should show you where the difference is...
hm, thank you, but i can't see any difference.
i will stay on 2008 R2..
That is correct, only Win Server 2012 Standard 64-bit is on the list.
As with all FileMaker products, this clause is listed:
*Version stated is the minimum requirement. FileMaker Server may also work with later operating systems certified by FileMaker, Inc. Operating systems not listed in the table above have not been tested and are not supported.
So it COULD work with R2, it's just not tested/support for it.
I got FMS13 to work on Server 2012 essentials one time (had to essentially delete IIS, but still got it to work). It's ALWAYS a good idea to use the recommended OS that is listed.