LR 6 was available as both subscription and for purchase. The final purchasable license was 6.14. From 7.0 forward it is available only by subscription ($10/mo in the US). I personally would not recommend buying version 6 now, it is pretty badly out of date, and things are starting to break (also in 5), e.g. the map module stopped (or is about to, I forget the date) working unless you are on version 8.
There is no solution to "Lightroom runs slow". There are only incremental improvements. Adobe has done a lot, but depending on what you do (lots of brush/local adjustments are indeed often a problem), and what your expectations are, there is no "fix". Some people see how fast it runs as just fine, some (and I am one) will never be happy if I ever have to wait for anything during editing -- and even in the latest version on a fast computer I still wait. That is why discussing "it runs slowly" is so hard online, it is very hard to distinguish "broken" from "all you can get from that computer" unless you have some kind of side by side comparison you can do with a friend's similar computer.
Do try the GPU setting, see if it will "take", see if it helps. If it is already on, try turning it off (in rare cases it makes it slower). Maybe the clear-preferences, though be sure you know how to reset them to what you need first (I've seen that help people, but remain reluctant because it fails to help frequently and some people get really confused with all preferences reset).
A new computer will absolutely help. Will it help enough? I don't know. A new computer might require a subscription (I do not keep up with Mac and which versions run on which LR versions, sorry).
Any camera clubs or such around, where you might compare with other users, literally side by side?