I'm not sure what actual keybind it is, but it happens to me sometimes when I adjust my headset volume. Usually adjusting it one more click in up or down will toggle it back off.
Yes, I can confirm that now - I just muted my computer sound, and guess what? The same problem happened again, instantly, just as I pressed it, totally out of the blue. And then re-enabling sound volume did indeed toggle the "slow motion" effect back off... however, the game was still being glitchy, so I shut down the game and launcher and re-started it all from scratch. All seems fine after doing so. Weird problem, I'm playing RR3 Mainline on AT Launcher if anyone is wondering, it's probably tied to that modpack for whatever reason. Is that the pack you're playing when it happens, Inaeo?