The way this project is set up circumvents the collision cells optimisation. The cell size is the viewport size, so in this case by using a totally impractical size of 10000x10000 (honestly, surely nobody would ever choose that in practice?) forces all collisions to be made within one cell. So then you get the worst-case of all possible collision checks, plus the overhead of managing collision cells. That's probably why you're seeing it as slower.
If you set the viewport size to something reasonable, it runs far faster. Even with a viewport size of 1000x1000 it runs several times faster for me.
If you want a full view while using collision cells, set a reasonable viewport size, and then set a small layout scale so you can see everything. That way you'd get the same view but the improved performance too. I did this for some of the screenshots at the time. Also collision cells are an absolutely critical performance optimisation if used sensibly - in many cases they can completely eliminate 90% or more of all collision checks, which in many cases completely eliminates the performance overhead of collision checks. It's such a huge optimisation that it's definitely a good idea to work with it, not against it. Also I have to say reporting an issue about a change that was made over three years ago is not very helpful - the sooner you can make a report, the better.
Closing as won't fix.