If I only have one device connected to it, is the speed as fast as if I plugged it in directly to computer?
It depends on the hub. But assuming the hub has its own source of power and operates at the fastest transfer rate the device and port support, then yes. Otherwise, it could force the device into a lower power mode or a lower transfer speed.
If I have 2 flash drives plugged into and copy data simultaniously to my computer does the data alternate or do they both get transfered at the same time? (my usb flash drives max out at a read of 5 mb/s and i know usb can go much faster (20 mb/s or so))
The flash drives transfer data internally at the same time. The USB bus alternates. This is one of the reasons devices have buffers. While the total transfer rate from both devices can't exceed the port's speed, you will see both devices transferring at their full internal speed thanks to buffering.
If i copy data from 1 flash drive to another (both are plugged into the hub), does the data go to the computer or does the hub just redirect the data?
It goes to the computer. The data can't be redirected because it isn't the same on both devices. Filesystems don't just store data, they store metadata as well. They have journalling rules, buffer flushing rules, block sizes, and so on. There just isn't enough smarts on either device for it to talk directly to the other -- neither device understands its own filesystem let alone the other device's filesystem.