Page load, painting, uploading, everything. It's all very very slow. I use Notion very heavily at work, and until March of this year personally. In early 2020 I switched to Roam and I can't go back now. Roam is so amazingly fast it's like they're cheating or something. I wish every app would adopt the way Roam does it. Load everything up front.
But, everything is slow these days. Aside from Roam, HN and maybe a handful of native iOS apps, I can't really name any app/site that I use on daily basis that isn't painfully slow.
Roam takes a while to startup. Once up, it's quite fast to switch between pages. A bit like an IDE. It's still browser based I think, and there are no native apps for Android and iOS. So I feel constrained when I'm not at the desktop.
Notion mobile apps are quite good, so I never have to tell myself I'll do something when I go to my desktop.
It solves the same problems for me. Because Roam is almost like a markup language, you can make it do a lot of what Notion does. Especially if you use Notion purely as a personal wiki + task manager.
Notion is much better for businesses IMO. Like I said above, I use it for my startup. Specifically for task management, and even for our public docs site, ex: https://docs.haekka.com/
A bit of everything. Nothing is SUPER bad, it's just always, always a little slower than you'd expect/hope. UI lag and page load are the big ones. Not terrible, just...you can tell it's a big pile of JS written by a team that has prioritised features and graphical design over raw performance.
