[content assist] Investigate why default completion list is slower than VSCode Java when completion is enabled for all keys

When completing same code in both VSCodeJava and Eclipse, Eclipse JDT completion popup is slow to be populated compared to VSCode completions. This needs to be investigated and improved to provide same or more better level of response time in JDT.

This could be because of lack of debouncing in the JDT.UI editor for keystrokes.