typical 900x900 has the file size of 100-200kb. If you want some details about the products, you need a big picture, right?
and 100-200kb image is nothing today's internet speed, took less than seconds to load, right? Beside, lets assume you have 5 additional pictures (total 6 with parent picture). Once the page is loaded, only the parent picture gets loaded by the page, the other pictures are loaded AFTER they are clicked, so having 5 pictures won't increase the page loading time at all. Basically either you have 1 picture or 6 pictures, the page loading time will be the same. For this purpose, you can do this:
have the parent picture small (the regular size as you have now) so you won't be increasing page loading time. But have the "additional" picture be large, so if the buyers click the additional pictures, they will wait until it gets loaded and they will get some detailed view. Makes sense?
Regards.