Downloaded iOS 4 for my iPod touch (latest Generation) and fired it up.
I can't detect any increase or decrease in performance. My iPod Touch functions as it always have, with the same amount of speed, battery life, etc.
Yes, it multitasks according to the basic definition. Juggling is probably a better definition as it seems to keep things in the tray until they are needed or pulled up to the screen. From the home screen, you can double click the home button to reveal a tray showing every app you've opened recently. I assume these apps, if they had the functionality built into them, could run while you were in another app. Pandora is the only App I've got right now that does that like a champ. Everything else was designed push functionality, which is kinda messed up.
When you have the tray revealed, press and hold one of the Apps like you when you want to rearrange them. It gives you the option to kick them out of the Tray. Basically it's a quick way to stop an App without thumbing over to wherever it happens to be on the home screen.
No more instant messaging on my iPod Touch for awhile.
iBooks is fantastic on the iPod Touch. While it pains me to say this, I hope it functions this good on the iPad after it receives some iOS4 love.
Being able to set wallpaper on the home and lock screens is cool, and like the iPad the firmware update installed some stock stuff for the purpose.
The new folders option is snazzy. Just drag one App into another one to make a folder, at which point it gives you the option to name it. Then drag more Apps into the folder, all of which appear on the thumbnail as tiny um... thumbnails.
The Mail App got a nice makeover. It looks better and it takes a few less gestures to reach your account information. It also has an option to see all your mail from all your accounts in one window. Super Good.
Photos seems to have some new functionality as well. There's a tab at the bottom for Faces, Events, and Albums I don't recall seeing before.
Calculator got a new icon.
All in all I'm very pleased and can't wait for it to come out for iPad.