Searching in multiple buffers or directories can be slow. Searching can be greatly sped up using indexing -- e.g. using lucene.
The only issue is that indexing must be done ahead of time. What should be indexed? Suggestions:
1) Current open buffers
2) Last 'n' directories on which a directory search has been done
3) Current project
For (3), this feature would be most useful if jEdit had a proper "Search current project" feature that integrated with the ProjectViewer plugin. Then all the files in the current project could be indexed, and the index updated as files in the project changed.