I saw this and filled out the survey. My LR performance has been terrible. On a new-ish, 27" (NON-5k) iMac, with 24GB RAM and smart previews already built, I still typically have to wait 4-5 seconds to scroll pictures in Library module as the dreaded "Loading..." appears.
One the one hand, I'm glad they are finally acknowledging the issue.
On the other, I can't believe it has taken so long. There have been speed complaints for years. I remember when they were flipping to a subscription model, lots of people said the sky was falling but I said no, give them a chance. Seems like the joke is on me. Near as I can tell, they have moved 80% of the engineering onto other things and the 20% that are left have been pointed mostly to LR Mobile, adding camera support, and the occasional random feature (dehaze: moderately useful; the abandoned "easy import" debacle: useless).
I do love the application and hope they turn it around but I'm finally at the point I'll look at alternatives.