So for a non-developer that wishes to collaborate, they now need a GitHub license? That's a deal-breaker right there for any org that is composed of more than developers.
Something is seriously hosed with your configuration if it takes up to 30 seconds to load an issue, especially on your specs which can probably support 5000+ users. Multiple servers won't fix your problem...probably should have support eyeball your configuration.
I feel this is how Jira took off initially in orgs when they had a really affordable unlimited user tier. When hooked into LDAP/AD, suddenly every employee in the company was a user.
SaaS is a disaster without controls. They make it damn hard to automatically disable users who are no longer active. Then you have users picking competing duplicative tools, and now users can't work together without having duplicate accounts in each tool. Once they get you in, you've got no leverage--they'll just keep increasing prices because you're locked in. Companies are pushing you from perpetual license to cloud not for your benefit.