If anyone has seen my posts in the past, you'll know that I'm not a fan of the ACA, but disregarding that, let me be the first to say that this probably isn't the fault (or, at least not solely the fault) of those building the web frontends.
In Maryland, our exchange website is poorly designed and written, just on the frontend (I obviously can't see the backend), but at the same time, all of the frontend issues could be fixed with a clever caching scheme.
Where the real bottleneck almost certainly lies is when the system takes your user submitted data and has to post it into what is surely an old, legacy federal system so that it can verify your identity. That old legacy system may have seen upgrades in preparation for this, but probably not -- even so, there's simply no way to prepare for the onslaught of users accessing (indirectly) what was certain to have been an isolated, government-only database in the past.