Changing JRE of Execution Environment is needlessly slow

For example in the platform workspace changing the JRE freezes the UI for ~ 17sec.

There seem to be multiple reasons. Biggest:

1. 10 seconds to touch all projects:


2. 5 seconds in UI thread: to repeated scan the whole JRT for **every affected java project** in workspace:


The result (list of files in the JDK) could be shared between projects.
If that happens also during eclipse restart it might be worth for an optimization.