A doubt on cabin aircon

I've been on aircraft departing from Dubai during summer and wished I
wasn't before engine bleed air starts running the aircon, because for
some reason the cooling doesn't seem to have quite the same effect when
running from APU power, at least on the A3xxs.

Since this is certainly not something psychological, why was I feeling
uncomfortable? It isn't as if doors are kept open either, since
boarding is via airconditioned telescopic chutes, not ladders.

Ramapriya