I upgraded from 1.53 to 1.5, then 1.6 and now 1.7, and this problem always remains. 

Now with more users were much slower. 

I have downloaded all the files 1.7 full package, I think the update pack had not installed all. 

Remains unresolved, only warnings attached image that is.

These are notices you can ignore them However, first three are related to antispammer plugin, if you disable it, they'll go away, you need to contact plugin developer to resolve the issue. 

as to the last one - probably you did some typo, check your config.php file

I entered www.yourdomain.com/ow_cron/run.php and I get 500 internal server error. This has nothing to do with the software it is server related issue. 

All plugins are updated. 

I disabled antispammer. 

With debug mode to true when you run: 

misite.com/ow_cron/cron.php 

I only get "Page not found" 

developper and debug mode to true and the same result. 

Remains unresolved, hundreds of users are active.

Quote from ross can you please disable this auto friends plugin and see if the problem persists.