I think L9 is a wonderful gift for some older machines. I hadn't updated my older laptop for some time and I decided to see how it would run L9. I have a very large library so the update was a real slug timewise. However, L9 runs a whole lot faster than L8 once you get past the indexing.
My suggestion is that if you want to make more regular use of an older machine, you probably want to be a bit more selective and load a smaller library from scratch. That way you spend less maintenance time and just use the machine as a study resource more than a full exegetical battleship. For now, my older machine is a backup, but I think I will switch it down to resource use later if I keep it.
Win 10 and other software improvements like L9 have greatly increased the potential life of older computers.