Ah yeah. Disqus. They never answer calls, and you can't have Single Sign On. They offer it, they just don't respond to your requests. So our clients won't accept Disqus any more, but there's a gap in the market now for a commenting engine with good single sign-on handling and syncing to the local install.

WordPress performance *is* an issue. Out of the box, with some caching, it's OK. Quick enough to match static sites on posts and pages. But, erm, that's about it. But the problem starts with the dynamic side when dealing with high traffic, high content sites. You can DoS a large WP site quite easily by simply putting in some awkward, randomised search queries, for example. No amount of caching will help there. If you use Elastic Search or Google CSE or similar, then no problem. But if you don't? Site's down. Damn nuisance. And there's a lot of that in WP along with little motivation to solve the problem because Automattic, who drive the project by and large, have a solution as part of one of their products - VIP Hosting.

Is this a problem? Dunno. But if you don't want to buy into Automattic's VIP service and don't want to learn and pay for all the workarounds the maybe there are better solutions than WP for certain use cases.