Apologies if this has been covered before (or indeed placed in the wrong forum) but I'm curious about this. Since dabbling in resurrecting some of my old Mac hardware, my natural preference is to install the latest OS on every computer. However, quite often this isn't the smartest to do. Handoff, continuity, Siri, security - count these aside. For a pure performance optimal hardware / software combination, what does everyone here do with their legacy, their ageing, and their current Mac hardware: do you run the latest OS or do you have a preferred optimal or original OS for your machine? Examples: 1) 10.12.6 runs horribly on my MacBook Pro 17" from 2011 despite having a 2.2Ghz i7 and 16Gb RAM 2) My 2007 Blackbook was painfully laggy with 10.7.5 despite having support for it - for me, 10.4.10 was always the best, most optimal for it. 3) I run 10.12.6 on my home iMac 27" (2010, 20Gb RAM, 256Gb SSD, 2Tb HDD) and have no problems with it whatsoever.