Just wanted to share some information in case anyone else is currently having this problem…
I don’t know what exactly caused it, but after a large system upgrade following the renaming of KDE5 packages I found that waybar was acting funny. I’d login, but it wouldn’t load automatically. I had to run it manually in a terminal - and even then - it would take 25-30 seconds for it to open. But once it was open, it was fine.
After much investigation, I it turned out I needed to install sdbus-cpp. Once I did that, everything started working as normal.
I can confirm this too. On first log in after boot waybar takes a long time to come up, but if you log out and back in waybar comes up fast. Did install sdbus-cpp , and waybar loads quick after a reboot and first log in
This link is what pointed me in the right direction. I found that libsdbus-c++.so.1 could not be found, so I installed the package where it was included.