If you are using the JEE edition of HELIOS, the assists can be speeded
up by turning off JAX-WS assist in the edit/advanced preference
(Window->Preferences->Java->Editor->Content Assist->Advanced.
It also appears that the assists cycle through a list of suggestion
sources, starting with a different one each time, which would explain
why the assists are not always slow.
See
https://bugs.eclipse.org/bugs/show_bug.cgi?id=317979