Globally disable threading for these for now.

It doesn't win us much performance-wise (as it turned out), and causes
weird crashers with large documents.

Change-Id: I0566ce850b8f6656e6f9821c653b7a825a7874a3
(cherry picked from commit e0425f9493e25fa14f11505d9ebc12a2fe78d4b6)