Ther nas no lak, but that he was agast
To love, and for to speke shamefast.
- Chaucer, The Legend of Good Women
Since this use of fast fell out of use, the second element in this word was reanalyzed as faced.
fast meant and still means "firm, fixed" and was from Proto-Germanic *fastuz, from Proto-Indo-European *past- "solid, firm". The verb fast "to abstain from food" is from the same PIE root, by way of Old English fæstan from Proto-Germanic *fastējan "to hold fast, observe abstinence". Breakfast is from break plus the Old Norse verb fasta "to fast", also from *fastējan.
The adverb fast shifted from meaning "firmly", as in stand fast, to "stoutly, strongly vigorously"
Tristrem as aman, Fast he gan to fiȝt
- Sir Tristrem, c1320
And then to "quickly".
Takens, war-thurgh he may understande, þat þe day of dome es fast comande.
- Hampole, The pricke of conscience, c1340