There's a noticeable lag in curves since version 21.1. I am now on 21.1.2 and it's still there. Up until 21.0 they used to refresh smoothly.
I am attaching a video to illustrate the problem.
I'm also attaching a couple of screenshots of my performance tab in "Preferences", but nothing changes when I allocate more RAM anyway.
These are my PC specs: Operating System Windows 10 Home 64-bit CPU Intel Core i7 7700K @ 4.20GHz Kaby Lake 14nm Technology RAM 16.0GB Dual-Channel @ 1070MHz Motherboard ASUSTeK COMPUTER INC. PRIME Z270-A (LGA1151) Graphics 4095MB NVIDIA GeForce GTX 1050 Ti (Gigabyte)
Sorry to hear that there's a lag with Curves setting in Photoshop, have you tried resolving the issue yourself?
Also, thanks for sharing all the relevant info with us, could you please try enabling "Legacy compositing" located in Photoshop's Preferences > Performance, then relaunch Photoshop and let us know if it helps?