As robotresearcher says in a parallel comment chain, FAST mode was without the screen running. Normally the ZX81 only did anything during the vertical blanking, most of the time it was drawing the screen. If you enabled FAST mode then it could go ten times quicker or more.
Sure the ZX81 sold internationally but it was so much more successful in the home market than overseas.
Sure the ZX81 sold internationally but it was so much more successful in the home market than overseas.