The code to insert items into the lookaside table was changed from using WT_CURSOR.insert to using WT_CURSOR.update, with the expectation that WT_CURSOR.update would search any currently pinned leaf page before searching the tree. It was thought that would be a faster point update for the lookaside table as it's generally appending to a single page at the end of the tree.
That wasn't correct. Because the lookaside insert code "sets" the key (using WT_CURSOR.set_key), any pinned page information is ignored, and a full tree search is performed.
Because WT_CURSOR.insert holds resources for a shorter period than WT_CURSOR.update, we should switch back to using WT_CURSOR.insert, it should make lookaside table operations somewhat less resource intensive.
See commit 30c226f.