MacOS – Mountain Lion – Flash sluggish, spinning beachballs etc

I upgraded from OS X 10.6.8 Snow Leopard to Mountain Lion 10.8, and then to 10.8.2 Mountain Lion.

I only have a few installed applications (Photoshop and Microsoft Office), VLC movie player and Firefox – the rest of the system is intact. All fonts required for Photoshop are installed within Library's Adobe/Fonts folder so all Adobe apps can use them.

However, I've had spinning beachballs on using apps, and choppy video playback when using VLC – it did not do that in 10.6.8 with any DVD or video played; and my previous movie player of choice (MPlayer OSX Extended, now uninstalled) was not compatible with Mountain Lion as far as I'm aware – it just crashed or I had to end it.

Adobe Flash Player does not work, well, except for some Flash-based sites and Google Maps etc – Streetview, and flash-based adverts on Firefox, but it's a bit on-and-off, working on some sites but not others like YouTube – where I get the sound, but not the video, only the spinning grey wheel, and then Firefox crashes.

Safari won't open multiple tabs either… one tab at a time, unless I open a new tab from within an existing site (tried it on bbc.co.uk today!)

Why are these 'beachballs' happening, as I don't have many files, the system Fonts folder is intact, the Adobe fonts folder only has a few fonts [Adobe ones and a few free ones I downloaded] and there aren't that many apps.

I ran Onyx, repaired disk permissions etc. but don't understand why this is happening – should I just go back to OS X 10.6.8 as my machine had 10.6 (it's a 2010 Mac Mini installed with 10.6).

The disk's got 500GB in total (can't remember how much is used, but the size of Photoshop CS5 alongside VLC and Firefox is… well, I'm not sure how much GB is used).

New to Mac and OS X in general, so would appreciate the help I can get.