I've just committed patch #3130, which makes it possible to set the minimum of characters to 1.
The default is still 3, so the current behaviour should be unchanged.
About the slowness, yes there is slowness if the setting is set at 1 and I select 'e' in codecompletion.cpp for example.
It take 1-2 seconds to select all the occurrences. But generally it works OK.