What was you machine like when it was new Paul? I bet it was fast as heck 
I think the point many of us are making is Apple may well be deliberately throttling older Macs because they want people to upgrade. The slowness on my machine for example, there is absolutely zero reason for it - the machine has 64GB of RAM a good GPU, CPU, SSD etc. There is not genuine reason why it should take 30 seconds to open an app, where the UI continually stutters, etc.
Sory about the rant 