Why is the speed up to 153 floating point numbers?

Yup, max speed is currently 500 in each direction by default. It’s easy to add an api for changing it (it’s currently a parameter on the PhysicsEngine constructor, but there shouldn’t be any risk to making it adjustable without creating a new one as it’s only used at the beginning of the movement during physics.) The new physics post I made when we updated to include it has more info / an example of how you can change it for now: New Sprite Physics in beta!

Edit: Wrote up a bit more reasonable ts api: https://github.com/microsoft/pxt-common-packages/pull/974