Speed display is incorrect when the belt speed is higher than 100 ...
When set to 30 per level, the speed display is
30, 60, 90, 1.2e + 02, 1.5e + 02, 1.8e + 02 respectively
Why is this?

30, 60, 90, 1.2 * 10^2 = 120, 1.5 * 10^2 = 150, 1.8 * 10^2 = 180. Looks perfectly correct to me.

One could argue this formatting switch happens early relative to 3 figure numbers needing 9 digits to be represented in scientific notation but I wouldn't say it's "incorrect" and it'll work just fine if you were to jack the setting up higher.

Yeah... it's probably the game doing this because it's never really been tested in 3 digit speeds. Maybe you should post this bug in the base-game bug reporting section?

Also, keep in mind that the game only handles belt speeds correctly in multiple of 1.875 (It's something to do with 60 ticks per second, and a tile being broken into 256 steps internally, it can only move an item an exact number of steps per tick)