Unfortunately, WHMCS is popular because there aren't many great alternatives that are production ready. If you think WHMCS is bad for the end-user, just wait until you use providers who use Hostbill or Blesta. Much worse end user UIs in my opinion.
While we're working on replacing WHMCS sometime next year (due to their annual price increases) we've not yet tested anything that's even remotely as good.
Performance wise, I think a lot of that has to do with how people are hosting it. It is very bloated and heavy though, especially once you're rocking with a big DB and a lot of actions to be completed at the daily cron (Generating and sending invoices, auto-suspend overdue accounts, auto-terminate really really overdue accounts, etc). Even on a relatively large server we hit random snags related to MySQL with some frequency... Just part of it, I guess.
Thank you for the detailed response, that makes a lot of sense from a business point of view in regards to panels like Blesta, they are no so feature rich but as a user not a hoster they are often more enjoyable due to fast action times / load times on the dashboard. (Although I assume harder to skin as i've not seen a good one)
WHMCS costs over $60/month correct? so over $600/year in that single piece of software, i'm not sure what the developer charge in this space but in the gaming space, a panel will all functions that WHMCS does for your business wouldn't be more than a couple thousand. Projects in Go and Laravel make a large portion of the work trivial a hoster does not need 500 payment options, and 25 VM manager intergrations you use one? So my curiosity to questions why someone like yourself hasn't hired or paid a developer.
Finally one more thing if you somehow have time, on this fine sunday is how people present WHMCS, WHMCS actually has a wonderful API instead of spending the time theming the panel, it's API could be leveraged for a modern proxied interface with it's own queue system, end-user would hardly know it's WHMCS or slow for 90% of it's interaction.