Ethernet roundtrip time with NicheStack & Nios-II

A Cyclone IV is definitely able to make a Nios II system with Ethernet run at 100MHz. Did you specify correct timing requirements for Timequest? You can usr the timing advisor to check your project for configuration changes to optimize timing.

I also noticed that adding an Avalon MM Pipeline before the Nios CPU JTAG debug interface helps timing.

As for the 0.0.0.0 IP address, if you disabled the DHCP then you need to specify a static IP address. It is done in one of the source files, in a function called something like get_my_ip_address(). I don't remember the exact name, I haven't used the Interniche stack in a while.