Yes I notice my computer becomes very slow when I have more then 5 tabs open sometimes in IE8.
Yes I have.. with as many as 4 tabs, my computer runs as though I had more than 15 tabs open in Opera.
Though, the slowness might have to do with the fact that IE spawns a new process for each tab you have open. I know this because I had 8 of the same process, and when I closed one, they all, as well as IE, disappeared. This is one of the many reasons why I don't use IE, and it is why I'll always opt for Opera and Firefox instead.
I still don't understand why they felt the need to have it spawn a new process for each tab that is open.. Opera and Firefox don't do this.. and so I really don't understand their thought process behind this.
the simple answer is because they make IE to fail.