display a large image by nios

I think you'll need to be more specific if you hope to get any assistance here.

It also sounds suspiciously like a school project ("...I must use a nios..."). No commercial project would have such a requirement; you use what makes sense to do the job. Having a CPU streaming from a frame buffer to a video output - inside an FPGA - is just, well, silly.

You can of course access SDRAM from a NIOS. Speed also depends on how you clock your design, caching, software architecture etc.