When slow queries are logged we include information about documents and index entries scanned to help diagnose performance issues. However that information doesn't tell us whether the documents or index entries scanned were in cache or not, and if they are not in cache they can have a much more significant performance impact. Reporting bytes read into cache for each query logged would help diagnose the cause of performance issues related to cache pressure.
This might be accomplished by tracking bytes read into cache per cursor or per session in WT, and computing the difference between the value of this counter before and after each query. Query performance impact would need to be evaluated.