i get 2800 credits/min and ~770xp/minute from it using a 787B in AFK, apart from clicking start and collecting the rewards ofc. times are also exclusive of load times, and that's without skipping the last race.
with my X1 i get about ~4200cr/min and ~1100xp/min.
try using my calculators mate.. it's what i use to calculate this things. 👍
by the way: i think A-Spec gives significantly more rewards than B-Spec, but also requires more effort. don't ask me though about the exact figures in A-Spec tho as i haven't bothered much of it past professional. i almost exclusively do the grinding in B-Spec.