Another way to put it is as follows: what is the minimum time from a plane stopping to when an evac can begin (without running engines interfering with slides) assuming all the checklist items begun immediately and are executed without difficulty? Is this possible within 20 seconds? Or does it take 40+ seconds? If the former, it would appear that there was some delay in commencing or difficulty in performing the checklists.
There are many checklists. But if at any time the PIC feels that an immediate evac is required, then the Evacuation checklist can be executed in seconds. Typically set flaps / brakes / spoilers, check plane depressurized, cut engines and evac. That's it.
So any delay would not be because the Evacuation checklist is too long. However the pilots might not have felt or known that an immediate evac was necessary. They could have been going through a different checklist (e.g., Engine Fire checklist), communicating with cabin, ATC and fire crews, etc., etc.