Large multisite install: out of memory errors after upgrade to 4.3.3


we upgrade to 4.3.3 yesterday, now when viewing the site list in the Network Admin dashboard we are getting the following fatal error at the time it has to render the top level site information...

PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes) in /var/www/html/wordpress/wp-includes/wp-db.php on line 2125, referer:

We've obviously googled a lot - this doesn't appear to be the split cron bug that affected the 4.3 release (this was patched in the 4.3.1+ releases).

The site list table always stops at the "" top level site at the user count column. The top level site does have a lot of user registered to it - but we never experienced this issue before the upgrade.

Struggling to find anything on this - don't want to just blindly increase memory limit as it's already pretty large - this just masks the problem really.

Need advice!