..the speeds seems to be normal, the limitation is the spi_clk speed (max 9MHz on your board) and fcpu, sandisk is the fastest as I can see some results (see http://www.microchip.com/forums/m574511-print.aspx with some nice pictures on comparision of various sdcards vs. fstypes vs. spi speeds, run on pic32 - but the mcu does not matter here)..