I would imagine that the speed brakes act like vortex generators which re-energise the boundary layer. (possibly a venturi effect along the ends?).
Without teaching anyone to suck eggs...this delays airflow separation.
I attached vortex generators in front of the ailerons on a 1960s glass glider which increased roll rate at low airspeeds.
I also flew another "hot" ship which had them mounted on both sides of the fin.
What was never explained to me was why it was not allowed to use wing mounted brakes with flap? (HS 121/ VC10 DC9/10).