Lightroom has always been a pretty frustrating application. A truly excellent concept with truly awful performance. Just getting into the ballpark of tolerable requires that you use a very fast multi-core processor and have multiple fast disks and tons of RAM. Anything less and you will want to commit hara kiri on a daily basis.
But even with a monster machine, turning on people mode will make you want to throw things at your screen. Seriously.
The user interface acts like it was written by high school students. I cannot detect any separation of GUI and worker threads and the areas of sensitivity and responsiveness make no sense whatsoever.
Even when performing it a folder at a time, the performance never gets any better.
You can’t put it in the background to run against the whole library because even with its thread priority lowered it will cause your mouse to jerk from place to place quite often. It is able, even at low priority, to make your system feel like it is from 1988.
I just wish that they would do a little professional coding and keep the user interface responsive with notifications and status attached to each face so that you can know what the system thinks is happening at any given moment. With the proper use of low priority threads, this thing could be absolutely amazing chugging away while you work on other stuff.
Who writes code like that?
And if you want to see how many people agree … take a look: https://forums.adobe.com/message/7475201#7475201