Late 2009 iMac running slow

Are you running any anti virus or cleaning software? Consider downloading and running EtreCheck http://www.etresoft.com/etrecheck and posting its result. That'll give us a good snapshot of your system and hopefully provide some answers.

Not having any idea if software is the culprit, I'd suggest running Disk Utility to repair the startup volume and check the SMART status.

Having your computer suddenly slow down isn't a security concern. Performance problems on a Mac are almost never caused by malware.

The biggest concern here, especially if this is a 2009 machine that has never had its hard drive replaced, is that your hard drive is dying. If it dies - and it could die instantly at any moment - it will take all your data with it if you don't have backups. For this reason, the number one top priority needs to be making sure you have a good set of backups. One backup is not adequate. You will need a minimum of two separate backup drives. Preferably, you will also want to create those backups with two different programs. I use Time Machine (built into Mac OS X) with a Time Capsule and Carbon Copy Cloner with several external hard drives.

Once you've got your backups in place, you can start trying to troubleshoot the problem. See my Mac Performance Guide for some suggestions.

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Who uses these other User accounts?

DId this start happening immediately after creating the new user account or after that new user account was getting used?

Could be something that the new user/s installed or downloaded onto your Mac?

Another Poster had a smiler issue to yours and that Poster found that the the other user of the computer had downloaded things that had over a short time was making the Mac slow.

Does this user have the ability to download and install any new applications to the OS X Applications folder?

Check your Applications folder to see if there is anything installed that you do not recognize.

See:

Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X,
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.

In order to repair the hard drive you have to be started up from your recover partition by holding Command R at startup and run Disk Utility from there.

Running as an admin user on a Mac is not the security risk it is made out to be. In order to really embed anything in the system, malware still has to get you to enter the admin password. Besides which, malware can do things just as nasty and steal your data just as easily with no admin privileges. A lot of the malware out there these days doesn't even try to get admin privileges.

But that's all moot, and doesn't help you with your problem. The problem with a dying hard drive is determining that it is actually, in fact, dying. If Disk Utility shows that its SMART status isn't "Verified," that's definitive evidence that it's dying. In practice, I've rarely ever seen a case where SMART status gave advance warning. If Disk Utility reports that it is damaged, that's generally evidence that the drive is dying, but a drive may be able to be erased and reused successfully for a long time before the drive finally gives up the ghost.

If there's no definitive evidence that it's dying, it can be a difficult call. If the disk really is 5 years old, it would probably benefit you to replace the drive, but there's always the chance that replacing the drive won't cure the problem, and then you're out a wad of cash that you didn't have to spend.

Soooo.....how we doin with that EtreCheck idea? I'd still like to see the report.