|Thanks Jarvis for his helpfull answer NOTTTTT|
As security pro I beg the differ.
IE is not rock solid but Firefox cetainly is NOT EITHER.
But enough of that, it is no answer to her Q.
You have probably upgraded to the latest IE version lately and found out that IE was lagging. (what IE version are you running ?)
How much RAM memory is there in your computer.
For IE7 or higher 1 GB of RAM is needed to run it smoothly.