MK (and most fighting games that i know of ) run at 60 Frames per second (Fps)
1 frame is equal to 1/60th of a second.
If subs slide was active for 50 frames before patch (made that up) then it was active for 50/60ths of a second.
+10 frames would mean subs slide is now active for 60/60ths of a second, also known as 1 second.
Bear in mind that frame data regarding subs slide was made up.
Although i have never heard of frames being used to calculate distance, what i think has happened is you have mis understood.
I think subs slide got an extra 10 frames of recovery, so if someone blocked a slide post patch they now have an extra 10/60ths of a second to punish it before sub can recover and block, meaning characters with slower moves, or players with slower reactions, have more chance to punish a sub slide .