This is most impactful in places where the struct being queried in Lucee is not a regular struct but some other object like a scope. For instance in *RequestService.cfc$getContext()*:
Just changing this line to StructKeyExists() saved tens of megabytes in certain requests in our application.
StructKeyExists( request, "somekey" ) uses almost zero memory in my testing, where as request.keyExists( "somekey" ) takes around 10kb. Not huge, but can really add up in certain types of requests. The BIF is also faster.