Likewise I have departed BKK 4 times in the last month. The 'fast track' includes diplomatic, APEC, crew, Thailand Elite, those departing using the airline fast track gold cardboard card, and those with small children.

The latter group is new in my experience. As WLGBase notes, it really isn't as fast track as it used to be - but still better than the main departure (I assume).

After check-in when facing the various departure options (i.e. south), those departing in F/J on THAI have a dedicated fastback that provides direct entry to the THAI lounge area in concourse D (IIRC). I usually use the fastback in the centre of concourse D. I'm not sure if there is another further to the 'right'.

Cheers, TK