When I bring up the sidebars, the whole app gets very slow and choppy until my sidebar has been fully brought in. This is less pronounced when I have a project open with few notes but the more notes (and attachments), the worse it gets. Interestingly, the problem goes away completely when I make Agenda full-screen.
This problem has been brought up before here: Agenda is pretty slow
and here for iOS: Clunky animation in sidebar (iOS)
There’s a number of technical reasons for this, while we agree it’s suboptimal, we also feel that spending a lot of effort for our two-man-team is better spent on other parts of the app. Noted though!
I can see that, thanks for the heads-up 
I feel like there are more places like this where the app feels choppy. I should note that I’m generally very happy with it overall (been a user for five years now and wouldn’t look anywhere else) and am currently not missing any features so you did a really good job with the vision and execution behind Agenda, kudos!
I hope there will be some point in the future where you will tackle this. Maybe make a one-week hackathon just for performance issues? Wishful thinking here 
Why this is a challenge in Agenda is that it presents a whole project of notes in a single list, where other apps show only a single note. When you resize a window, the notes have to be re-laid out, their heights all calculated etc, and it needs to do that many times for the animation to work.
It would probably be possible to make this more performant, but we would need to rewrite the whole logic of the note list replacing the standard Apple layout. Unfortunately it’s not a one week hackathon, probably a project in the months.
We’ll take it along. It would certainly be nice to do the rewrite, because it may help with other things too, but for now, we have projects that bring much requested new functionality.
Thanks for the feedback!