I haven’t noticed any issues, but I just ran a test. A file from my IQ3 100 I added over 100 spot removal locations, some drawn, others simply clicks. While it seemed slightly less responsive by the end, it really was insignificant. I reverted the file, did several global adjustments, as well as add some graduated filters. I again used the spot removal tool to create over 100 locations, again with no perceptible change. I then added some adjustment brushes and created some local adjustments, and even when using the automask function I had no problems at all.
This is on my laptop, after I had been working on several images for the past couple of hours, without relaunching LR. Lightroom, the files, and the catalog all reside on the Laptop’s internal SSD which does offer pretty astounding performance (write speed 1600 MB/s, read speed almost 2000 MB/s).
I do know that if I have done extensive work on a file before I do spot removal that can get laggy. Not uncommon for me since I usually am trying to decide how much time I want to invest in a file so I have several adjustments, some graduated filters, and often two or three extensive brush adjustments. In this case I make a virtual copy, revert the file, do all the spot removal, then sync the spot removal over to the other file.
Your experience that one machine performs better has me curious because it seems this is better than what I’ve seen on my MacPro with 64gigs of ram where I do most of my LR/PS work. I think I’ll test it tonight. The main difference is LR and the files are mounted from my Laptop using target disk mode, with far less speed, but there maybe some underlying differences which is causing such a diverse range of performance for users (which may or may not be something Adobe can actually do something about).