You see Borg drones or cubes being attacked and while some are being destroyed, they quickly adapt.
But HOW do they adapt mid-battle?
And how is technology described so they can assimilate it?
Every programmer knows that you can rarely just copy paste foreign code. The compiler will output error messages.
Research is not just brain power - which they have enough of due to the collective - it is also experiments and building things.
At least experiments need time.
Optimization is a field of its own in engineering aimed at making foreign technologies fit.
And they, too, need experimentation.
They might have time to assimilate, but they don't have time to adapt.
Is adaptation supposed to happen mid-battle?
When a ship is attacked or a drone?