In my experience MC13 is faster than MC12. There must be a reason for the slowliness you are experiencing.
Could you explain how exactly MC13 is slow? Is everything slow or does the problem show up only in certain areas?
What kind of media files do you have and how many of them? Are the files on a local drive or are you accessing them through network? If you use a LAN connection, how fast is it?
Thumbnails are done in advance (so i don't think it should be a problem)
This is not possible. You can't import MC12 thumbnails to MC13. The storage system is different and in any case the library restore/conversion would reset the old thumbnails.
(EDIT: naturally this does not apply if you mean the manually started "build thumbnails" process by "done in advance".)Thumbnails are small cached copies of cover art. If you have video files without separate cover art MC creates thumbnails on the fly by running a bit of each video and taking a screengrab. Depending on your files and file locations this may take more or less time. AFAIK, thumbnailing is a low priority background process, but MC cannot fully control the external OS level video decoders and other DS filters.
BTW, did you let MC13 use an existing library (or restore an MC12 library backup file) or did you import your media files to an empty library?