I've converted one of my larger files (40,000 persons, 30,000+ images) and while the program is slower than 2010, it is faster than 2012 on my 64-bit machine and uses memory much better.
Knowing of other's issues scrolling through images in 2014, the first thing I did after conversion, was to allow the program to regenerate the thumbnails by slowly scrolling through the images. Once this was done, scrolling performance was at least 2012's scroll rate. and let the thumbnails generate (FTM 2014 uses a new method for these images and has to make new ones.)
I've stayed with 2010 for my large trees (2 of which total 80GB data), but am pleased enough with the lack of memory issues in 64-bit 2014 that I've converted and accept the slowness as compared to 2010 for now.)
Edit: I do not use Sync on my large files.