EZ-HOST is USB 2.0 Full speed controller.So the theoretical maximum value will be 1.5MBPs=12Mbps.
Practical throughputs will vary around 200KBPS - 1MBPS .It depends on mostly firmware overhead which largely depends on type of applications like mass storage-CF , MP3 , IDE interfaces, Class devices. To list some of the factors of firmware that affect throughput:
1. Standard USB request processing + Class requests (mass storage,audio , printer class etc)
2. Communication interfaces like
SPI Slave: 1-2 Mbps
SPI Master: 12 Mbps
UART: 115 Kbps
With slower communication interfaces, the processor should share the load between interface and the USB bus. This will reduce the throughput drastically. With faster communication interfaces the throughput will be near to max limit. In this case it depends on the user code design here to get better throughput.