I haven't had the checkout shipping page loop, but Canada Post does take a long time to respond.
I have a canada post shipping module in both my Zencart and another cart, and the other cart takes about 4 seconds and the zencart takes between 5 and 30 seconds depending on how many products are in the shopping cart.
It is very frustrating. I am trying to establish my website so it will keep my inventory, but that means that it calculates 16 oz as 16 individual one ounce bottles. You get a few 16 oz bottles in the cart and it can really slow down.
grrr
I haven't figured out exactly how to get around this, but I am hoping to figure out how to put a "please be patient" pop up so my customers will not jump ship thinking my site is stalled.
Pat