That's a good idea! I may decide to go with compounding SPEEDups, but I'm also considering ascension-type mechanics as well as a time warp functionality which would supplement the SPEEDups. Basically at this point the game is still in an alpha state though so there's still lots of things left undecided at the moment.
As a counterpoint: one of the goals I had for this game is to make upgrades that can still be bought frequently throughout mid-game to be able to increase engagement, so by having non-compounding speed upgrades, a player can keep tweaking things without buying one compounding upgrade, then having to wait a long while for enough currency to buy the next compounding upgrade, then longer for the next one, and so on. For me, when playing similar games, I find myself getting bored when I buy what I think will be a huge efficiency upgrade only to then have to sit for a half hour or more to wait for enough currency to buy the next one.
Granted, buying one SPEEDup to go from say 14x speed to 14.25x speed isn't massive, and at a certain point even visually you can't tell the difference anymore (like the 5 second counters onscreen cycling all the way through in just a few frames) but at least it feels like I'm doing something.
Your feedback is much appreciated though, thanks for checking out the game!