TT was/is old-style "slick", like at the time it put my bank to shame in terms of UI. It probably wasn't using a 'popular framework' and if it was, it wasn't a naive implementation. It also was/is optimized for desktop, while many financial corps now only care about mobile. So as of last April, it was still good enough for me.

Was it fast? No. Just fast enough. That's why I doubt the desktop app is really any different. They must have a bunch of API endpoints, and the 'slowness' is all on the backend.