HTTP or HTTPS does not affect speed. It will only affect caching of the page which does not affect the first time load anyway. HTTPS is necessary for us as we don't want a malicious user on someone's network to steal their login information.
I tested the link with Opera, FireFox, Safari and IE and it seems to load just fine. Our admin panel load is just 110KB, so even for dial-up connections, it won't take more than 4 seconds.
It could have been that at a certain point, due to some issue with our internet provider the site access was slow maybe.
Does it open slow for you still?