After we upgraded our test wordpress multisite to 3.5 we realized that that both the ‘admin’ user and the other siteadmin users where suddenly added as a ‘participant’ to all the wp sites we had.
Siteadmin shouldn’t be a member or participant to any blog in a multisite wordpress installation we think.
The biggest problem was that the working as ‘admin’ or as a siteadmin user with 100-200 blogs attached, made everything go really slow. 10-15 seconds per click in average.

We found that bbPress was causing the issue, because we could do a test when the bbPress plugin was not installed, nothing happened. After the plugin was activated, sure, the same problem happened again.

We want to dig more into to the issue. At this moment bbPress plugin is disabled in our wordpress 3.5 Elvin system. And it will be also disabled when we upgrade our main production system.