The big problem with the B737 is that the wing is quite high to allow pax to vacate. At the gate the flaps are up, selecting the lever to F40 uses the electric hydraulic pump to move the flaps, this takes some time. If the APU is being used to supply electrics, then one of the following items of the EVAC checklist is to pull the Fire Warning switch, thus powering down the hydraulic pump.
From memory, I think the checklist asks the crew to wait for flaps to reach 40 if possible. I think with fire/smoke. Most people would press on and get out.
Ryanair had an evac in STN many moons ago after landing and ran into the same type of problems, the fire crews actually instructed pax to go back into the cabin from the wings!! (it was bearing failure/oil fire from the engine)