I bought the StarTech dual 4K DisplayLink adapater after watching one of those videos. I use it to power a second 4K monitor with my M1 MacBook Air running lid-closed. I have a 1080p gaming monitor as my primary connected to a Elgato dock via DisplayPort.
DisplayLink is definitely suboptimial, but it's the only option. There is noticable input lag (my guess is around 100ms) that makes mouse and keyboard input feel sluggish. Showing a moving image on the screen pegs the DisplayLink Manager process to 100% of one core while it's playing. DisplayLink does not currently support rotation on Big Sur, though it probably will in the future. However, the image itself is pixel-perfect, so it's fine for secondary apps like chat or monitoring.
> There is noticeable input lag (my guess is around 100ms) that makes mouse and keyboard input feel sluggish.
That's interesting to hear. In some of these videos (and additional Reddit/YouTube comments on them) it seemed like a lot of people would say "yeah but it will have lag" and the person making the video would claim that it doesn't seem to while dragging windows all over the place in their videos. The rotation seems to be a problem across the board though.
These things are difficult to dissect. It could be that there is lag for these other guys, but they aren't the type to notice. Or maybe they didn't get lag because of some detail in their set-up that differed from yours.
Oh well. I don't even have two monitors right now and probably don't really have enough room for a second one. I had hoped to get one this year though.
Maybe I'll get the M1 and just keep working with the one monitor. If I decide it's time for an upgrade maybe I can go the route discussed elsewhere in this thread and get an ultrawide, or a single 40+ inch 4k.
EDIT: Video example of what I'm talking about:
No lag with a displaylink second display on the 2020 M1 MacBook Pro!
DisplayLink is definitely suboptimial, but it's the only option. There is noticable input lag (my guess is around 100ms) that makes mouse and keyboard input feel sluggish. Showing a moving image on the screen pegs the DisplayLink Manager process to 100% of one core while it's playing. DisplayLink does not currently support rotation on Big Sur, though it probably will in the future. However, the image itself is pixel-perfect, so it's fine for secondary apps like chat or monitoring.