The speed crits are not cumulative with each other, you only subtract the worst, although if that one is repaired you still suffer from the next worst. The speed crits are cumulative with half speed from being crippled, though, and the half speed applies first. So a crippled white star with -4 speed goes at (15/2)-4 = 3.5".
To be really pedantic, the weapon arc loss crits are cumulative too... you can lose more than one weapon arc.