Add new wiki in wp-admin is painfully slow

Whenever I select 'add new wiki', it takes up to several minutes to load.
All other wp-admin functions are not slow.
I have disabled all plugins one at a time and that made no difference
I changed to the 20-20 default theme and no difference.

Also very slow when use 'edit wiki' (backend), but not when use 'edit' tab on front end.

Any ideas?