Actually I believe it is working correctly.
That is the Speed value you pass in, is a value that says SPI shall not exceed this speed.
You did not specify much here, but if I assume a default build, the T3.5 was built at 120mhz? Which implies that the bus (F_BUS) was defined to be 60mhz.
The SPI clock is setup by using clock dividers. The two nearest values for clock dividers are:
128 so 60000000/128 = 468750 which is too big
and 192 60000000/192 = 312500 which is what you are seeing.