Sometimes, syntax checking can be a bit slow, especially when large packages are edited. It might be useful to add a little delay to the syntax checking Job (which can be seen in the Eclipse Progress view) in order to avoid syntax checking when typing fastly.

E.g. when hitting two keys within 100ms, don't check the syntax after the first keystroke. Or in other words, only check syntax when the editor was idle for at least 100ms / 200ms / some configurable value...