One can obtain faster results by using directly the Linux interface and not using the arduino API (which performs the GPIO multiplex setup for every call). Via the Linux API, one can perform the GPIO wiring during setup and read later from e.g. /sys/bus/iio/devices/iio:device0/in_voltage1_raw (see e.g. Sergey's Blog - Malinov Family Web Presence). Via Linux I was measuring 2ms for analog reads. For some applications this speed is fine, for some other applications a no-go. The problem with the is here mostly the GPIO expander (multiplexer) running at 100 KHz. The Galileo should be in general capable to perform much faster analog reads via an external ADC device connected via SPI by using the fast GPIO ports of the Galileo, but that is not an option for using the Arduino pins + Arduino API.
See some discussion here:
Galileo Board: Performing a sequence of analogReads faster? - Arduino Forum