I'm not quite certain what's the use case for most soft CPU's on FPGA-s. My experience is from the other side (Xilinx) and it's more than half a decade old, however looking at stuff it doesn't seem to me that things have changed that much. What 2 useful CPU designs I used were the PicoBlaze, a bespoke microcontroller with a weird architecture designed to take advantage of FPGA implementation details, and was ridiculously tiny, like a RAM block + a few slices worth. The other one was the ARM Cortex A9 core physical core on the Zynq FPGA.

MicroBlaze, Xilinx's own big soft-CPU core was a combination of slow and large, with a buggy compiler to boot.