If all you want to do is delete the restricted sites, you can do open Internet Options/Security tab.
Click the red circle with a line through it, then sites. You can delete them individually, but with 300 sites there, this would take a lot of time to do.
I personally don't think having that many restricted sites is slowing down your computer. When was the last time you deleted the cache? It's deleting cookies that wipes out your stored passwords. History, temp internet files, cookies can build up-causing a slowdown. It's a good idea to regularly delete them. As long as you know all your passwords, enter them back in when required.
Also, what about defragging? That might help with the speed issue.
Slow Computer?, Use this troubleshooting checklist
for more info on things you can do to speed up your computer.
One overlooked file that needs a regular cleaning is the Temp file. (not the same as temporary internet files). TEMP FILES. There are 2 files containing temps. One is temporary internet files, the other is the temp files. Temp files are created when certain programs are run or installed. (Windows is SUPPOSED to delete these files automatically but it often misses LOTS and these files, if left behind, can cause many problems with your computer.) Temp files should be deleted when the application using them closes, but many times they are not. Over time, the files build up, take up drive space and can affect your computers performance. These temp files are safe to delete. However, they do not automatically show up on your computer. They are in a part of the hidden files and you must configure Windows Explorer in order to see them.
To enable hidden files to show click on My Computer Icon. At the top of the page, Tools/Folder Options/View. Scroll down and check show hidden files and folder, apply, then ok.
To view them, you must open up Drive C, then documents and settings. If you have more than one user account on your computer, click the name of the account to open it, Documents and Settings/Local Settings. Not sure what you would see if there is only one user, just play around until you find it.
Hit ctrl+a to select them all, then hit your delete key. If it won't delete some of them, that's because they are attached to some program running in the background, so you may have to delete them one at a time. If one or two won't delete, they will probably delete when you reboot next, so don't worry about them.
Finally, do you have a desktop computer? Have you ever taken the case apart and cleaned out the innards? Dust can make the chips inside your computer run hotter and also clog cooling vents. At least once a month UNPLUG YOUR COMPUTER FROM THE POWER SOURCE, open the case, and blow out the dust. Do not wipe it with a rag; use either your breath or an aerosol can of compressed air. Also, DO NOT block the vents from your tower or the moniter.You can also use a vacuum cleaner with the hose on the blower if you have one, but whichever method you use, DO NOT LET ANYTHING ACTUALLY TOUCH THE CABLES, MOTHERBOARD, ETC. Free airflow is a must to keep a computer from heating up.
All this is way more than you probably expected, but I hope it will help you.