Summary:
This is a performance improvement and potentially an eviction improvement: transaction commit/rollback are too slow in the case of prepared transactions, and a reasonably large transaction commit/rollback of a prepared transaction (that is, one with thousands of updates) takes a long time. If the updates are on an underlying page that's big enough to be forcibly evicted (so, imagine creating a collection, inserting a few thousand documents, preparing those documents and then committing the transaction), it can stall eviction and lead to format test timeouts.

prepared transaction resolution can stall eviction on active pages

Original failure:
http://build.wiredtiger.com:8080/job/wiredtiger-test-format-stress-zseries/50671/