MacOS – Will Mountain Lion make an older computer run faster or slower

As Steve Moser says there is no support for 32 bit drivers and Mountain Lion will refuse to install on the macs which had 32 bits drivers (someone seems to be too lazy to rewrite them?).

If this answer is not the one you want to hear, then you can install Mountain Lion the hard way. Because there is a way (which worked for the preview).

You will need another compatible mac (support) and you will have to do some hard drive switching.

The idea goes as follow:

• Get Mountain Lion
• On the target Mac remove the hard drive (target drive)
• On the support Mac insert the hard drive (USB or internally)
• Install Mountain Lion on the target drive
• Put the target drive back in the target Mac
• Boot on the target drive holding Cmd + V And write down the ID given in the error message "Incompatible Mac detected: ..."
• Put the target drive back to the support Mac and on this drive in /System/Library/CoreServices edit PlatformSupport.plist
  • Add your board ID in the SupportedBoardIds section.
  • Add your model ID in the SupportedModelProperties section.
• Put the target drive to the target Mac
• Boot

You should be able to get to the desktop but without hardware graphics support.

To enable the graphics hardware support you will have to install the right kernel extentions. You could use kexthelper.

EDIT: To avoid all the hard drive switching and messing with internal components you can boot the target Mac in the so conveniently named "target mode" and connect it to the support Mac on which you could install the OS to the target drive and then boot on the target drive to edit the plist files.

As a rule, the OS takes care of itself. An upgrade both ensures the core OS is intact, but also disables code that is too old to run.

If you would have done a clean install anyhow - lots of slowness, crashiness, etc. Then that's the reason to do a clean install. In the past (10.4 and earlier) the install process was different than the migration assistant but as the migration assistant has matured, it now runs anytime you upgrade so unless you hand migrate files and preferences, there is no difference between a clean install and an upgrade in place (assuming you select apps and settings in the assistant).

I personally love a clean install and using all the defaults for a week. Consciously installing just what I need feels "right" for me and if I end up missing everything, I can always wipe and migrate the entire setup a week I to using the new OS.

In summary, for most users, the changes in the OS have disabled the worst offenders in terms of performance, and gatekeeper keeps most old apps from running until you explicitly allow them to run. If you have performance issues, you can always flip that switch and reboot to isolate the cause of slowness.