Parallels Desktop for Mac uses Mac system resources depending on the difficulty of tasks you are running inside Virtual Machine. System performance mainly depends on Mac available amount of memory (RAM). 512 MB of RAM on the Mac should be enough for using basic guest OS functions, but it's recomended to have at least 1-2 GB of RAM for running complex programs inside Virtual Machine. You can check your Mac memory amount by clicking on the Apple icon on the Mac menubar and selecting "About This Mac" option.
If you are experiencing low system performance with VM running try the following:
- Restart the Mac.
- When VM with some guest OS is running click on the "Parallels Desktop" menu on the Mac menubar, select "About Parallels Desktop" option, hit "More Info" button and check if Virtualization Mode (VT-x) is available. Generally you should be able to activate VT-x on your Mac by putting computer to sleep for 3-5 minutes.
- Launch Parallels Desktop for Mac -> click on the "Acceleration" link in the main configuration window -> set "Cache Policy" optimization setting to "MacOS X".