Slowdown with named snapshots

The mongodb test jstests/noPassthroughWithMongod/indexbg_interrupts.js takes significantly longer with snapshots enabled (42 min vs 1 min). Most of the time is spent in the convertToCapped command. Profiling shows that the vast majority of the time is spent iterating the update lists in __rec_txn_read and __wt_row_modify.

My theory is that the existence of a snapshot is preventing splitting a page. This workload creates a new collection and inserts ~100k records into it without creating new snapshots or retiring old ones. I think that the linear scans are causing this to be an O(N^2) operation.