Reinstalling Lion/Mountain Lion Without Erasing the Drive
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu.
Reinstall Lion/Mountain Lion: Select Reinstall Lion/Mountain Lion and click on the Continue button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.
Restore from the last Time Machine backup or clone of your preceding version of OS X.
There should be no reason for your recent MacBook Pro to be slower due to Mountain Lion exclusively. Mine seems to perform better. I used Geekbench which confirms a small, but measurable performance increase over Lion.
Something else is going on with your MBP that is not likely to be resolved by downgrading OS X. Find out what it is and fix it.
Quite often the slow down can be caused by incompatible software
or drivers that "get stuck" and and complete their task.
Best thing to do is first open up Activity Monitor and select
All Processes from the drop down menu. Then click the
CPU column to get the list in decending order to see what
may be using up CPU. If nothing shows up excessive,
click the Disk Activity to see if the disk is being accessed
excessively. Also, check network activity in the same way.
The other item is to also check memory usage and also
the Page Outs. Low system memory can also cause