Profiling on Windows on load bench showed OpenSparseFileIfExists at 6% of samples, and experiments with avoiding them shows major gains in at least 
hot reads, and likely modest ones with cold. 

Measuring with procmon suggests only ~0.02ms per entry, though, though this will only be in a few milliseconds for large pages, but that may be enough to try avoiding.

One way to do it is by keeping a flag in the _0 file footer.