How to know which usb port is high speed

That usually refers to USB3, but given the age of the machine it doesn't even have USB3. Maybe its referring to a USB2 port and your using a USB1?

My advice, test every USB port available till it stops giving you the message. Then leave the memory stick in there so it stops bugging you  .

What is the device you are plugging in (is it a usb 2 or USB 3 device).

Is this the System you have??
http://support.dell.com/support/edocs/systems/opgx620/e...
IF so, You have NO USB 3 ports, So Only solution if You want to use a USB 3 Thumbdrive at full speed you will need to buy a USB3 pci-e card.

If you are using a USB1 Thumdrive, Buy a new one as they are very cheap. If the Device is a USB 1 and Not a memory card/thumbdrive - Then you live with it untill you replace it with a usb 2 device (Falls in that sorry charlie catagory).
If You are getting this message when plugging in a USB 2 device, then most likely Your driver is not working properly. May need to update it.

For Drivers and Bios versions, See: http://www.dell.com/support/drivers/us/en/04/Product/op...