Mine got to stage where applications took so long to start and were so slow to run it was virtually useless. Sat in the cupboard for a few years then one rainy afternoon when I was bored and searching for something to do so I could look busy if the missus wanted me to do something, I removed the mac OS and installed Mint linux on it. Big improvement. Now its completely usable again
I had to buy a new Mac a few years ago due to a new (second hand) camera to process Raw files.
But I still use my 2009 Mac Mini running 10.6.8 the last version that Photoshop will run on. When the computer dies I will get something else to replace Photoshop as I am not going to rent it.
The old computer is not that slow because the software is not overly bloated. Also use a SSD with it for working files.
I also have PCs for 3D rendering.
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Procrastination, mankind's greatest labour saving device!
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