There are a few things that can be done to speed up config="" or config=NULL cases:

• On entry to API functions, if config is "" (empty string), we should set it to NULL.  Since our config system (should) handle both the same, it should behave the same, and the NULL will be a little faster on every call to get a config value.
• For open_cursor only, the cfg variable (the array of strings), can be set to NULL if the config argument to the API is "" or NULL.
• We already have a way to fast path "overwrite=false" (a common MongoDB case), we should extend this - basically set cfg to NULL after processing the overwrite.
• __cursor_reuse_or_init checks cfg to see if certain things could be fast pathed, but __wt_config_gets_def(session, cfg, "dump", 0, &cval is not fast pathed, but could be.

louis.williams@mongodb.com suggested the last two items.