Issue https://github.com/Alexey-T/ATSynEdit/issues/93 needs much work:
(slow part in replace: UpdateBuffer which is called after each replace in editor)

maybe a) make regexpr.pas variant which can read TStream,
b) make TStream over ATStrings (again use FStarts:TList), 
c) make new finder which don't use ATStringBuffer (and StrText var) 
at all but uses streams and directly reads-writes stream...