As a simple checck you could open a
cmd.exe shell and try the following command multiple times while waiting for the browser to open the page.
netstat -an -p tcp
Normally you should not get lines with
But, persistent existence of such lines while waiting suggests the site is taking longer to respond.
If this happens long enough, you could also try ping these IP addresses while your browser is waiting (within that 1-2 minute period you describe).
It would be important to differentiate that while the browser is waiting,
it is actually waiting for the site to respond and not just thrashing on your windows machine.
Another check: during this 1-2 minute time, does the browser stop responding for any site you try? or, is it waiting for just one specific site?
This will also help differentiate if its your browser, path to some specific site or your network connection that is having trouble in this time.
Update on your comment:
If its not browser, OS and web-site specific, it is very likely a glitch on the ISP end that sort-of stalls your connection for a while.
Since you move to Ubuntu and try Firefox when the problem hits, i suspect system load is also not related here.
If you are working over a direct Internet connection, so there are no Web-Proxies in the path,
it is odd that you can ping the web-site while your browser is stalled reaching it...
Maybe you can get a packet capture which has the ping working while you were repeatedly trying the same site from your browser.