FYI for the dev's.
I've seen quite a few posts in my research of this problem from others also experiencing it. My guess is the responsiveness issue maybe related to how DIA's editor loads/uses
Fonts.
Several years ago I used the DIA Diagram Editor a lot for diagramming networks etc. It was a great simple to use tool!
A couple years went by where I'd not needed to use DIA.
During that time Ubuntu 24.04 LTS was released!
In the past couple months I'd been working on a project that I needed to produce some diagrams for again and started to use DIA again.
But for some reason DIA's performance when entering/editing Text was really terrible, with long delay's between a "click" and an action.
I had been searching for any info on reports of this and any possible fixes.
Today I discovered the fix and it had to do with the massive amount of Font's that are enabled in Ubuntu.
So following the Tip I read & I installed the application font-manager.
$ sudo apt -y install font-manager
The fix...
Run font-manager and disable all fonts but the ones you actually use!
After that DIA's performance was great again for me!