Why is mac so slow

To start with you likely need to do some HDD housecleaning! A HDD can quickly get its data fragmented across the sectors causing the drive to work harder trying to piece the App or Data back into a readable chunk!

Think of it this way you have a string which you've strung a bunch of colorful beads. But you need to replace a given color for a different one of course undoing the string carefully not altering the order of the beads only swapping out the one color would be tedious! That's about what your drive is doing. But here it's more of reusing the freed space placing another data block within it which might we smaller or even larger so other spaces need to be used overtime the drive gets bogged down which is likely a good portion of your issue.

The second is not having enough free space on the drive, so depending on the drives size the OS needs needs to limit it's virtual RAM use as well as cache and then depending on the App it to can get bogged down as the scratch (temp) space it needs is not available.

So what to do?

To start off you need to make a full backup to make sure you don't loose anything, next setup an external bootable drive which you can run Disk Utility from (if you still have the OS installer disks you might be able to use it if it's the same version your system it currently running). Next is getting a good HDD drive repair App which might be hard as the older OS your system is running is quite old many companies have moved on. But give these folks a try Drive Genius you'll need ask for the version based on what your systems OS is. But before you can run the defragmentation tool you'll need to free up space getting rid of old stuff. A good file clean up tool can make quick work of it, again the version of your OS can be an issue here.

Lastly... Upgrading both RAM maximize it and you might want to upgrade the HDD but be careful here! The version of SATA this system has wouldn't run most of the current drives being made! You need a drive which supports SATA II (3.0 Gbps) not the current SATA III (6.0 Gbps) drive standard.

Perform a quick restart, as background apps and programs can slowly eat away at your RAM and thus lower your operating speed. If that doesn't seem to fix anything, this is an almost 10-year-old system, so I would strongly consider an upgrade to a newer system. The core duo is beans on toast by today's standards, and a newer CPU would help to speed up modern-day tasks that this Mac can't handle quite well.