Why is Blender slowing down? This is something I have never witnessed with any software before.
When I open Blender it works properly; I click on an object, vertex, etc., and it lights up immediately.
But after about half an hour of work it can take up to a full second before it lights up, and commands start to yield awkward results.
After an hour it can take up to 5 seconds before a clicked item lights up, and commands are yielding weirder results.
My processor is not being taxed in any way, nor is my memory.
I close the software and reopen it and everything works fine again. But after half an hour it slows down again.
Is this programmed in the free version to annoy people who can't pay for the pro version? Because I can't find any logical reason why the software slows down through time during usage.
Could anyone explain this?
The first thing that comes to mind is that Blender might get bogged down by the number of steps you have performed which it stores in RAM, in order for you to Undo your actions.
The default value for this is 32 steps, and you can change this in your Preferences:

As you can see, you can also set a memory limit to these steps.
There is no paid or pro version of Blender. There is no bad programming to pester people who don't pay for this software, there is no planned obsolescence - it's free and open source.