50K images is not all that large for a catalog.
How big in bytes is the LrCat file?
How much free space is left on the drive?
How much RAM does the computer have?
I'm on a Mac Pro (2013). I'm just throwing this out there as something to try. Prior to upgrading to 7.2, my catalog file was on an external drive (USB 3). After upgrading it would take FOREVER for LR to load. I moved my catalog file (roughly 1.9GB) to my home drive (/pictures/Lightroom CC...made the directory). Restarted and everything sort of came up. I say sort of because I had to wait an hour or so while everything was updated. I have roughly 99,000 images in my catalog. I left LR 7.2 running all night and today I haven't had any issues. Personally I think LR 7.2 "expects" the catalog file to be in your home directory, any other location it just struggles. Just as a confirmation, I forgot to save new location and when I started LR 7.2, it went into the spinning wheel of death while things loaded. Changed location in preference to new location and restarted. NO PROBLEMS as of yet. Also running 7.2 on my MacBook Pro (2010), with catalog file in home directory. NO issues.
Just throwing that out that. All it cost is the time to copy file to home directory.
HOWEVER, if your catalog file is already on the system drive home directory, disregard all the above