Hi,
I checked your website but the pages where no HikaShop stuff is displayed takes as much time to load as HikaShop pages on my end.
On my end, on a test website I'm using with HikaShop configured, a Joomla article page has 30 queries and a HikaShop product page with more than 30 variants (a lot more than on your website) has 60 queries and there is less than a second of difference in the load time, without any kind of caching.
Knowing all that, I don't see why the slow downs would come from HikaShop. Probably something else, but what, I don't know.
Now regarding the checkout prices doubling, you probably have that issue:
www.hikashop.com/forum/install-update/87...to-2-4-0/193295.html