It won't surprise me if it does lag.
The program is bigger and can do more things. So if your computer is slow than yes, IE7 will slow everything down.
One advantage Firefox have is, it's a smaller program and does not hog the computer. So using Firefox is slightly faster than using IE7.
You can tell by just checking the file size. If you download Firefox, it's less than 6 MB. If you dowload IE7, I believe it's more than 15 MB. I am not sure since I stop using IE long time ago.
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