Expansion does nothing as it
slows down your game even more doesn't affect the game speed.
The reason why the lag happens is because of something unrelated to the bowser fight: The music. Bowser has got multiple tracks and changes between them between phases.
This isn't a problem on the original SMW but it becomes a different story once the ROM comes in contact with AddmusicK.
The reason for the short freeze is technical: SMW has got all its level songs loaded at time (which includes all of Bowser's battle musics) but AMK only loads a single one. The waiting simply means the game has to load a different song which includes the samples.
The last point is the main reason for the relatively long freeze as all SMW songs shares all the samples. In order to fix that, you simply have to put the following code in level 1C7 to
UberASM:
Code LDA #$01
STA $7FB001
Keep in mind that one have to clear out $7FB001 manually. As such, I recommend you to put the following code on gamemode 11:
Code LDA #$00
STA $7FB001
Now, a freeze still happens but it is notably shorter than with the samples.