OK - This is pretty good now! I got something like 3577 (lost the screen when I logged in to comment) before hitting the space bar too soon on a double jump. I'm running Chromium 80 - Ubuntu 18.04lts - arm64hf - (Odroid XU4) and it seems pretty smooth. I'll go back and check Firefox & win10 shortly.
Kind of surprising / embarrassing how easy it is to screw up. My most common error is mistaking + for x or vice versa, but sometimes I just hit the wrong key, or subtract wrong!