Websites Suddenly Very Slow in IE, Fine Everywhere Else

On Friday our sites suddenly stopped loading in Internet Explorer (all versions), but still load fine on every other browser. This issue is affecting multiple sites in both OpenCart and WordPress. We have already tried several potential solutions and learned the following: 1. Increasing the server timeout limit allowed the sites to load in IE after 200 seconds, while they were still loading in less than 5 seconds elsewhere. 2. We copied one set of site files and DB to an entirely separate server and the problem persisted. 3. Hosting support says the server is fine, and confirmed that a simple index file with "test" does load fine on all browsers. 4. We also reverted the website files (not the DB) to a time when IE was working, but the problem remained. We have no idea why only IE is being affected and need someone who can quickly identify and fix the root problem. Please review the above notes carefully and reply only if you have solution ideas beyond those. Thanks!