Not sure how much, but loading large files is bound to have some performance impact.
I also noticed the following comment in defaultfontstrings_simplifiedchinese.xml:
Code:
<!-- Split out from GAMEPAD_MEDIUM_FONT so we don't have to generate outlines for large CJK fonts.
This currently saves about 100 MB of memory, but is only useful as long as we only use outlines with latin characters (usually numbers). -->
<String name="GAMEPAD_MEDIUM_FONT_LATIN" value="EsoUI/Common/Fonts/FTN57.slug" />
So maybe disabling outlines could help reduce the size?