Drawing video took up a very large fraction of the CPU cycles on the ZX81 so there was a 'fast mode' and 'slow mode'. Fast mode cut out video output when progra

Drawing video took up a very large fraction of the CPU cycles on the ZX81 so there was a 'fast mode' and 'slow mode'. Fast mode cut out video output when programs were running, slow mode left it on and ran at something like 1/4 the speed.