Eclipse Juno has a number of performance regressions compared to Indigo. Switching back and forth between XML and Java editors can be tediously slow even in Juno SR1.
The patch provided for bug 394588 solves the issue or at least significantly improves the response time of the IDE.
In my team, we have tested the patch successfully on Windows 32 bit and Ubuntu Linux 64 bit with Oracle Java 6 and 7.
The patch can be installed from a special update site via Help | Install New Software.