Too many devices
A USB hub divides the bandwidth available among the devices connected to it. Connecting multiple devices to a single USB hub reduces their performance. Do not connect too many devices to a single hub: try and spread them out among the available USB ports.
Devices that utilize a lot of USB bandwidth should be directly connected to a USB port present on the motherboard, rather than one on a USB hub. For example I have my USB hard drive and webcam running off a direct USB port while my memory card reader and USB charger are connected to my USB hub.
Readyboost in Vista
Readyboost is a feature in Vista that allows you to use your USB flash drive as system memory. Readyboost acts like a large page file on your flash drive and helps in boosting your systems performance. To use Readyboost you would need a flash drive capable of good write speeds. Choose a fast flash drive from brands like Corsair or OCZ. You may also use flash drives that mention Readyboost on their packaging.
Here are the steps to enable Readyboost on your flash drive-
1. Plug in your USB drive and select its properties dialog box from my computer.
2. Click on the Readyboost tab and select "Use this device".
3. Select the maximum capacity for Readyboost usage. The higher you set this, the better the performance.
Write Caching for external drives
If you are using an external hard drive that is always connected to your system, you can enable write caching on the drive to improve its performance. Here is how you do it-
Step 1: Select the device manager from properties under My Computer.
Step 2: Select your external hard drive from the list of hardware.
Step 3: Select Properties->Disk properties
Step 4: Check the Write Cache Enabled option and click OK.
This is not advisable for USB flash drives as a power failure can cause data loss on the USB drive.
If you have an old PC with USB 1.1 ports, you can purchase a PCI card which has USB 2.0 ports. In some motherboards, Hi-speed USB must be toggled from the BIOS. If Windows is indicating that a device is not operating at full speed, it could be a hint that you are connecting a USB 2.0 enabled device to a USB 1.1 port and taking a huge hit in performance.