TL;DR It turns out lots of bookmarks for remote servers slows things down.
My Konsole was slow to create new windows, taking 5-10 seconds. It didn't happen all the time but frustrating nonetheless.
Debugging was unsuccessful. I removed all my ZSH configs, disabled KDE window overrides. But, nothing. No answers on Google.
Eventually, I realised it only happens when my fileserver is offline, so I started digging around all the Konsole options and found the problem.
I had a ton of bookmarks in Konsole, several of which were for remote servers. I didn't realise Konsole had this feature as it's not something I would use. The bookmark shortcut is Ctrl + Shift + B, very close to the paste Ctrl + Shift + V. I can see myself mashing that a few times.
I was unable to edit bookmarks initially because I hadn't installed the
keditbookmarks package on Archlinux. Once installed, I could delete all the bookmarks (this is also really laggy with the server offline), and voilà!
Fast Konsole once again.