Apple Prores files not opening in preview nor quicktime. FCPX incredibly slow

Just purchased a 15'' MacBook Pro 2017, 2.9ghz, all seemed to be fine. Imported about 100GB of Prores footage and it was quite quick with the import, however, the files cannot be previewed or opened by quicktime. I can import them in FCPX at an incredibly slow speed and I can work on them only with the proxy mode.

There seems to be a problem with the Prores format with MacOS High Sierra or my computer, because all the other mp4 videos work fine, and the same Prores files can be easily opened from my Late 2013 13'' Mac.

Any solution for this?

Thanks,

Jack