Created db with 5M docs full-text indexed as follows:
Then do a full-text search:
Finishes in about 10 seconds under mmapv1, ran for a several minutes without finishing under WT before I killed it. It also uses (possibly a lot) more memory under WT than mmapv1.
It seems to be spending all its time in forceFetchAllLocs (itself, not callees), called from yield. This path is only exercised if storage engine has document-level locking, which explains why WT behaves differently than mmapv1.