Since I upgraded to the latest version of Lightroom I find it even slower than the last version. Once you start layering brushes it's painfully slow and I have a reasonably decent PC. Even worse is when it backs up, it takes an eternity and makes EVERYTHING else on the pc almost unusable until it has finished. Great tool but horribly slow.

https://petapixel.com/2017/07/11/adobe-admits-lightroom-slow-says-speeding-top-priority/

https://fstoppers.com/lightroom/adobe-looks-improve-lightrooms-abysmal-performance-version-201512-187908

Oh well, that's promising at least