I'm relatively new to the Mac world, but I have upgraded several machines: Macbook 2008 from Leopard -> Snow Leopard iMac 2006 from Tiger -> Snow Leopard Both of them are upgraded without erasing the hard drive. The macbook seem to go really fast after the upgrade, but the iMac runs about the same afterwards. However I've had a bad experience with iPhoto '11 upgrade... It really cripples my iPhoto and so now it's very slow and I don't spend as much time on it anymore. The question is... Does OS X releases usually slow down macs? I know Snow Leopard is a kind of a special exception because it's a kind of a performance upgrade rather than a feature upgrade. I'm just wondering if I upgrade to later OSes (10.7, 10.8, etc), my macbook can bear it (given that I have the minimum RAM and CPU requirement). I have in fact upgraded the RAM in all my machines. Hopefully someone who has been upgrading since OS X 10.0 can give me inputs... Cheers!