Old English 'faest' meant 'firmly fixed,' not quick — the speed sense came centuries later, a dramatic semantic shift.
Moving or capable of moving at high speed; firmly fixed or attached; (of a clock) ahead of the correct time; (of colours) not fading.
From Old English 'fæst' meaning 'firmly fixed, steadfast, secure, enclosed,' from Proto-Germanic *fastuz meaning 'firm, fixed.' The original meaning was not 'quick' but 'firmly attached, immovable.' The sense of 'quick, rapid' developed in Middle English, possibly through the idea of running fast as in 'holding fast to a course' or 'keeping close' (hence 'moving quickly in pursuit'). The PIE root may be *past- (firm, solid
'Fast' originally meant 'firmly fixed' — the opposite of what most people assume. 'Hold fast' means 'hold firmly,' not 'hold quickly.' 'Steadfast' means 'standing firm.' A 'fastness' is a fortress. 'Breakfast' literally means 'breaking the fast' — ending the period of firm abstention from food during the night. The speed sense developed in Middle English, possibly from the idea of holding