Obviously they were under a lot of pressure and it's easy to stand here and criticise, but...
...if my site is slowing down with load or usage, I'm not sure how you make the jump to "I should update my UI libraries!". Angular 4 isn't getting any slower, so best case is you've got some unknown performance bottleneck in your UI that is somehow causing 30s page load times, and which just happens to be fixed in Angular 6, and you don't accidentally add any new issues when you upgrade.
Conversely, it feels like if you're struggling with "slow load times" on a SPA, the first thing you'd do is open the network tab and see what requests are being made, to what, how often, and how long they're taking.
Grasping at straws does seem to be the right metaphor. (Or maybe the old chestnut about the drunk dropping his car keys in a dark parking lot, then looking for them under a streetlight, since it's too dark to find them in the parking lot?)
I'm happy for the team and it sounds like things are going great for them, but wow, that was an almost fatal bit of blindness. On the plus side, I bet everyone involved will check for inefficient database calls first next time. :)