Some good tips I'd suggest are:
- Before you start importing large projects, go to Projects and uncheck Build Automatically
- Import projects in the smallest sub-components you can. If the project is Mavenized, this means selecting the lowest pom.xml in the tree to import each time, rather than importing a large parent POM and all it's sub-projects.
- When you are not working on (as in, modifying) the source code of a specific project, close it by right-clicking and going to Close Project. This will release resources for Eclipse to use elsewhere. If you still need to see the source code of something without modifying it (for debugging), you can add source containers as you need
- Use different workspaces for different projects. Eventually it's quicker to have 2 working, fast versions of Eclipse than one huge slow one
If you can't split a project, you can also try to give Eclipse more memory, and make sure you're using JVM 8, for example
-Dosgi.requiredJavaVersion=1.8 -Xms512m -Xmx2g
It sounds like you're just dealing with something that's too large for most any IDE, but that may not be the case. If all else fails, why not try IntelliJ to see if that works better for you?