"I just had a try and it took 40 minutes to solve the analysis with shell pair idealization (1536 elements) and 20 minutes without idealization (5342 elements) ."
This is due to the constraint equations that Mechanica needs to create to link the shell elements to the solid elements (those pink lines you see between the two different element types). Multipoint constraint equations are very expensive from a computational standpoint, but they only become an issue if you have a significant number of them. Given the fine mesh level at the interface between solid and shell elements, it's not surprising that the shell element model takes longer to solve than the solid element model.