Hi
Which is the actual U8glib constructor you use?
U8glib supports HW SPI with the DUE, but these pins are only available on the ISP header.
Another cause for a slowdown often is to call analog read inside the u8glib "picture loop". Sequence should be:
a) read analog value
b) store the value in a variable
c) execute picture loop, output value from variable
Maybe you can also post your code for further discussion.
Oliver