PicoZed is populated with a Micron 4GB, v4.41 MTFC4GMDEA-4M IT eMMC device.
Xilinx has a Design Advisory related to eMMC and performance: http://www.xilinx.com/support/answers/59999.html. Essentially, it states that the Zynq-7000 SDIO Controller operated in High Speed Mode is NOT compliant with the JEDEC standard 4.41 for eMMC. The only way you can run in High Speed Mode (50 MHz) is to add tuning to your board traces or verify with the eMMC manufacturer that the characterized hold time is 2ns or less.
Avnet has verified with Micron that the characterized hold time for MTFC4GMDEA-4M IT is well under 2ns. In other words, though the 2ns is outside JEDEC standard specifications, the Micron eMMC can still comply and function with the Zynq-7000 host SDIO controller in High Speed Mode.
This specific characterization data may be obtained under NDA with Micron.