Probably doesn't effect performance much.
If you want screaming speed, try using the "sgrep" utility in the Build window. It only support active code page stuff (no unicode) but for many of us, that's all we need.
sgrep main -t "*.cpp" "*.h" -- recursive
sgrep main -i -t "*.cpp" "*.h" -- ignore case, recursive
sgrep main -w -t "*.cpp" "*.h" -- match whole word, recursive
sgrep main -r -t "*.cpp" "*.h" -- SlickEdit regex, recursive
...
I use this for many of my simple multi-file searches. When I need color coding (which I sometimes do) or my search is more complex, then I use the GUI.
In the future, we will be making the GUI multi-file searching faster. It will never be as fast as sgrep though.