I've been on FTTP 500 for a couple of months. Consistently almost instant response to requests across wired and wifi devices. Yesterday I started to get delays on all devices, including last night when streaming through SkyQ box (Paramount+); the first time this has happened since I dumped Virgin for BT.
I then rebooted the BT modem and my own Mesh router, but get same result. Sometimes it's ok, sometimes not. If I run a speedtest it takes 10 seconds or so to start and then when it finally starts it gives circa 500Mbps (I just tried on a wired desktop and 21 seconds finding server then bang, 450Mbps download, but ping 226ms. Then another with 5ms ping)
So it appears that it's not the download speed as such, but the initial response to a request.