1.5.7.18 slow tabs?

[irix]

I have quite some tabs open at one time. Most of the time I switch from one to another tab, and so on.
With the 1.5.7.18 update when I go back from one tab to the other the page will be blank for 1-2 seconds, before the content comes back.
In the previous version the page was always there directly.
What could be the problem?