How to improve Fmax for SOPC System?

Note that Nios/SOPC performance is dictated by the FPGA performance, and is therefore not "solvable" to become faster. Compared to soft processors by other vendors or soft processors in other FPGAs, Nios is minimally equivalent and often better. Naturally, I don't know what you're doing or what your critical path is that dictates the performance, so it's hard to say. There are a number of examples and documentation on the website, such as using low-speed bridges to help performance, using custom instructions, C2H, etc. I think there are some useful posts on the website too. Good luck.