Pending registry approval is the status of a domain after customer has approved the transfer. Basically the old registrar can accept or reject the transfer. If they reject it they have to give a good reason, for instance that the customer hasn't paid bills, that there is a dispute going on, etc.

They can't do this just because they are vindictive etc.

No registry I know of will actually proactively accept a transfer.

Therefore what happens instead is that they'll let it time out after five working days. At that point the domain transfer auto-accepts

For that reason this takes 5 working days.