When you say UHD disc, I guess you mean 4K BluRay discs.
And when you say BluRay discs, I guess you mean 1080p (aka 2K) BluRay discs?
In any case, they both are BluRay discs.
It does take long as MC seems to scan all the playlist files first when you press "Play" and maybe does some other things like looking at the existing m2ts or whatever. It's taking less when trying to play a disc whose contents is stored on the hard drive, as access is faster than to an optical drive, but it still takes about 10-15 seconds or so (or even more).
I don't think this is unusual, it always did so.