Backup your files, erase the hard drive, install an earlier version of OS X.
Downgrade Lion/Mountain Lion to Snow Leopard
1. Boot from your Snow Leopard Installer Disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu.
2. After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Note the SMART status of the drive in DU's status area. If it does not say "Verified" then the drive is failing or has failed and will need replacing. SMART info will not be reported on external drives. Otherwise, click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the partition scheme to GUID then click on the OK button. Click on the Partition button and wait until the process has completed.
4. Quit DU and return to the installer. Install Snow Leopard.
This will erase the whole drive so be sure to backup your files if you don't have a backup already. If you have performed a TM backup using Lion be aware that you cannot restore from that backup in Snow Leopard (see below.) I suggest you make a separate backup using Carbon Copy Cloner 3.4.1.
If you have Snow Leopard Time Machine backups, do a full system restore per #14 in Time Machine - Frequently Asked Questions. If you have subsequent backups from Lion, you can restore newer items selectively, via the "Star Wars" display, per #15 there, but be careful; some Snow Leopard apps may not work with the Lion files.
You can't really uninstall your system. If you're asking about downgrading to a previous system, that requires that you erase the hard drive and either restore a previous system from a backup or reinstall a previous system from scratch.
Of course, if you're having problems with Mountain Lion, it would be easier to let us know what those problems are so we can help you solve them. Mountain Lion should work well.
As per your request, here's my problem.
I upgraded to Mountain Lion on my Macbook Pro (purchased last year) and have encountered problems with YouTube and other streaming channels ever since. Everything is slow, even after hours the videos don't load on YT. I'm watching everything in a stop/go/stop manner.
Cant' fix 10.8 slowness, that has to do with the bloated OS on older hardware, or a over 75% filled boot hard drive/lack of optimization.
However there could be a software issue.
Run through these Steps
Then if all is fixed then consider this
Cant' fix 10.8 slowness, that has to do with the bloated OS on older hardware
Hogwash! A MacBook Pro purchased last year should be perfectly capable of running Mountain Lion very well. It runs very well on my MBP purchased more than two years ago.
PunchiAuntie, it would be ideal if you would start your own topic for this, as the current topic is not the best place for you to get your particular issue addressed. However, I will say that you may find some help in my Mac Performance Guide. Beyond that, things that you would need to test would be checking your MacBook Pro's performance on different wireless networks, testing in a new user account and testing while started up in safe mode.