FOR ALL THE PEOPLE WHO ARE MOVING TOO FAST TO THE RIGHT, HERE'S HOW TO PATCH IT:
You'll need the Lunar IPS patcher program, the .IPS file linked here and and an original Super Imposter Bros ROM file. Open Lunar IPS and click "Apply IPS patch".
Select the IPS Patch File (click to download), and afterwards select the unmodified Super Imposter Bros file. Once you're done with those steps, the program should say "Patch applied".
Once you've done that, the ROM should be patched, and your movement speed should now be stable, for whatever reason!
Lumpy, if you see this, feel free to publish the patched file yourself, no credit is needed. I'm just happy to help!