User account performance and setup

Question

Hi,
I have a MB Core Duo 2006 model (old I know!) and have tried using user accounts in the past, but have been concerned by the hit they may have on performance (my MB has been slowing down a lot recently). My wife and kids all use the same computer and I have resorted back to everyone using the same account as Fast User Switching had become very slow. Ive just installed an SSD however, and now FUS has sped up, so Im considering whether to start using user accounts again as the benefits I feel could be great.
So, what Id like to know is, if one user leaves loads of applications and programs open and running in one user account, and a second user comes along and needs to use the computer and fast switches to their account, what sort of perfomance hit can be expected? Or does the activity on the first account all get paged to disk as it were and therefore does not occupy any RAM/processor time? If there is a performance hit, what sort of level are we talking about and what can be done to minimise it?
Further to this, my wife and I currently both share the same user account's iTunes for wifi syncing our iOS devices so we can both access the pool of stored apps, media, etc. that we have built up over the years. Is there anyway of ensuring that regardless of which account is open at any one time, all iOS devices will be synced correctly to our single iTunes library as I dont want to have to maintain multiple iTunes libraries??
Finally, how does Time Machine deal with constant user account switching?
Many thanks,
Rich.

Answer 1

Can the split of resource shared between background and foreground accounts be managed by the user in anyway??
I don't think there is any easy way to allocate the Mac's resources, and I wouldn't be playing with its deep processes anyway, but consider this.
Modern computers are powerful out of the box, and they have oodles of RAM (potentially) and gigabytes of hard disk real estate to stroll around, so I don't think a lack of resources is a real concern.
I read on x704.net that the Mars Rover is running on a 200 Mhz CPU from the late 90's! the same used in the Apple G3.
Modern Macs are pretty powerful to handle more than one user.
The most resource intensive activities on the CPU are file transferring and processing parrticularly large media file sizes , and internet  streaming of video which is heavy. Apart from these activities any other activities can be run simultaneously without too much drag.
It is not the amount of data loaded in RAM or paged out to Disk, but rather the CPU cycles devoted to a task that will slow things or crash the computer.
I used to play SecondLife and sometimes played two characters from two accounts (FUS) when I wanted to give some resources from one to the other, on the one Mac without any slowdown, despite SL being a resource intensive game.
If you add more RAM Rich, I think your Mac should be able to handle more than one account open. Computers can be pushed, their only stress is heat, for there is no friction to deal with, broadly speaking.
With sharing media, look at this link for one method.
iPhoto: Sharing libraries among multiple users