Slow shop plus chosing different product sizes takes a full reload

[kamstrup]

I have been having quite a few issues on our site www.netdyrlaege.dk
Firstly I even after optimizing a few things, still have the issue of "time to first byte" being crazy big seen in webpagetest.org. But that is a Joomla issue, I'm sure, not Virtuemart.
However, since a part of the optimization was to enable caching, omitting virtuemart though, product pages are still fairly slow.

And I really do not know how to proceed.

Take for example
http://www.netdyrlaege.dk/hund/frontline-combo-netdyrlaege

Here I have different sizes to chose, but every time one changes from one product size to another, 5 seconds or more pass.
So not only does a customer have to wait for this to load, they even have to wait to chose the right size. This is not acceptable user experience.

I really hope someone here have suggestions on how to improve this!

I'm on J2.5 and VM 2.
Server is at an advanced vps at hosteurope.de, just moved this week in order to try to increase page speed.
As far as I can tell I'm currently using 2% of the CPU.
So to the "blame it on the server- you need to upgrade" comments, well, maybe, but first I really have to try all possible settings and hacks, before this is an option. 

[jjk]

I'm on J2.5 and VM 2.

Which exact version of VM and how many products do you have approximately?