SPI_FULL_SPEED for reading SD card data

Having interfaced an ATmega2560 to an external SD card, I need to find out the speed of reading and writing blocks of data to the SD card via SPI Full duplex. Currently, all the File operations that I used is via half duplex with ATmega as Master with the standard pin cfg.

50 MISO
51 MOSI
52 SCK
53 SS

I want to log the timings for SPI R/W operations on the ATmega for full duplex mode where there are 5 slave devices.
Could anyone who has had successful experience for full duplex mode successfully share their experiences?

Sorry this post should be in the networking section.

What has this got to do with bootloader issues?