This is the 200th most frequent Swedish word.
fast
The Swedish word "fast" means "although" or "though" in English.
'Fastän' is used here as a conjunction meaning 'though', introducing a clause of concession.
Jag gick ut, fastän det regnade.
I went out, though it was raining.
'Fast' here is an adjective meaning 'solid' or 'firm', describing the state of the door being immovable.
Dörren var fast och gick inte att öppna.
The door was solid and could not be opened.
'Fast' in this context emphasizes a comparative phrase, 'even faster'.