According to the CurseForge description,

Speed Cart - The speed cart is a special cart that does not collide with mobs but comes with an incredible top speed (Configurable). This means that it cannot be pushed at high speeds, interrupting its travel.

However, when riding the speed cart, I found it can still be stopped by running into a cow (both with the CurseForge version and with a version built from the newer code in the repo). Sending an empty cart varies between picking up the cow and stopping before it (whichever cart I use; the different modid values make it possible to have both versions installed concurrently).

Side note: the "cannot be pushed" part seems valid, but only for the CurseForge version, and not just at high speed - I can place it on a track, and even when it isn't moving at all, I can walk through it instead of pushing it.