My experience has been different. Could depend on the building and the squad positioning in it? I didn't test it in a vacuum but when I used a 222 with a mortar I cleared a maxim out of a building repeatedly very quickly.
I had tested 222 and ostwind(i don't know exact spelling). they had no problem. Mortar is not too bad. Only centaur and flak halftrack took infinite time.(100~200 seconds or more sometimes)