It may not be due to something yo are doing, but how the browsers work.
Though some problems in a site can slow some browsers more than others.
For example in this topic, a 404 on a linked css caused a massive slowdown in Firefox only.
If you don’t have any such problems on a site, that it’s probably not something you are doing, but is down to the browser developers. If your scripts or pages are inherently slow, they will be slow in all browsers.