I was able to minimize the performance issue by adding a specval. No way to just isolate an area for gentext update.
<specval attname="gentext-only" attloc="system-var" attval="#ANY"/>
So it ends up like this:
<att><specval attname="gentext-only" attloc="system-var" attval="#ANY"/><fillval attname="TLMLinkend" attloc="SYSTEM-FUNC" fillcat="usetext" fillchar="source"/><charsubset><usetext placemnt="after"></usetext></charsubset></att>
Thanks to everyone for their help on this!