I believe the SGS-1-35 (and others) had the same/similar airfoil. They did not seem to do well with a waxed surface, even worse when wet/bugged.
Sorta along the lines of the dimples on a golf ball, a little roughness reduced the huge drag at airflow separation. Better a little when/where you want it, than a huge trip somewhere else.

An old trick was to clean the wing, then sponge down with liquid soap to about the spar. Then, if it rained, the water sheeted instead of beaded. The L/D dropped, but not as bad.

Good example, years ago at Mifflin PA, from the SE returning home, a flight of about 6. A couple ASW-20's, I believe Ventus, an LS-6 and a 1-35.
8 miles out, we hit a little rain. The glass ships all got marginal on final glide, the 1-35 sank fast compared to us. We crossed the last ridge OK, the 1-35 had some trouble.

As to blow holes, they are more like a "variable turbulator strip", the amount they trip the airflow is roughly linear with the airspeed.