I have around 2200 websites in my instance.
When I try to signup to openx market through the plugin I takes really long - it is because after signup user websites are sent to pubconsole and it looks it take a long time.
The problem might be that most users will get PHP timeout after 30 sec (default in php.ini) and still the account will be created - yet they will not see any confirmation (luckily the email is sent) and will probably try again with same data but will get "Already associated" error
It took over 10 mins on my machine to see the "thank you for signing up" screen.
Also, looks like it causes timeout on plugin upgrade in installer as well (see screenshot)