I've run into similar issues with Internet Explorer 11 on Windows 10, especially when working with JavaScript-heavy sites. You're definitely not alone in this, as IE 11 is notorious for its handling of modern web technologies.
One thing that worked for me was ensuring all the latest Windows updates were installed. Microsoft occasionally releases patches that improve compatibility and stability for IE. Also, you might want to check the "Compatibility View" settings. Adding some of those problematic sites to Compatibility View has helped streamline performance in certain cases.
Given IE's aging architecture, it struggles with newer web standards, which could be why JavaScript-heavy pages act up. If possible, try running those legacy applications in Edge's Internet Explorer mode, which might provide better stability and performance without needing to switch browsers entirely.
If you're open to exploring further, consider reaching out to your IT department to see if transitioning to a more updated browser environment could be feasible for those legacy applications. Let me know if you'd like deeper insight into any of these suggestions!