Are you just talking about the time it takes to lock when the resolution changes? Or is there actually noticeable lag.. like, when you move your mouse around or type?
The sync time does vary a lot between devices. I've had monitors that differed substantially how long it took to sync between its own VGA and DVI inputs. My TV is a little slow, so I might miss the POST screen on a fast-booting PC. But the ones that do RAM counts or floppy seeks always show up.
I will try my OSSC on a couple monitors tonight, if I have time. It seems to be pretty quick but I haven't ever paid much attention. This is something the retro console people care about, as some games would switch between 240p and 480i, for example when going into menus on Chrono Cross. On a CRT, there's no perceptible change. On a lot of digital TVs and scalers, it takes several seconds to transition. IIRC, the OSSC is relatively quick -- but it will also depend on how long your monitor takes to change modes.
The OSSC is VERY low-latency once locked, though -- which is part of what the device was built for. The HDMI output only lags a few scanlines behind the analog input. There's no full-frame buffer, just enough to do line doubling / tripling. Still too slow for Duck Hunt (plus no electron beam), but if your monitor were to render straight to the panel with zero overhead, then it would feel exactly like a CRT.