This standardizes prepare time for ranged units. Prepare time is the time between an attack order and the arrows actually flying.
Archers go from 500ms to 800ms standard.
Javelineers go from 750ms to 400ms.
This means, since everyone now uses 200ms turns, that Javs deal their first attack 50% faster than archer, which makes them easier to micro, reducing the impact of their more limited range. I think it's a buff to Jav cavalry in particular.
I think this is a natural counter to their greater range, too.
This also stnadardizes all jav units to have 1250ms repeat time, some had differences.
I could see making cavalry archer have 1000ms of prepare time, but that might look a bit odd with their 1000 repeat rate.