no that slow performance is not caused by missing out IE7. It must be other issues.
I would start by looking at the installed add-ons in IE, such as BHO's, (Browser Helper Objects), ActiveX, 3rd party toolbars, (Google, Yahoo, etc), and other plugins. Disabling them all, testing, then adding them back in onr at a time and testing again would be what I would try.
I would also scan for malware.
although I use Firefox most of the time I do use IE as well and wanted to upgrade to IE8 I only have IE6 installed at the moment the problem is when I install IE8 it is so slow to start and slow to use is this because I do not have IE7 installed or is this normal for IE8 I never have trouble with Firefox.