No clearly defined information.

We recommend 40MHz or more for debugging using JTAG.
Although it is an experimental level, I was able to set the JTAG Level to "No Debugger" and run it at 10KHz with the Cyclone III Starter Kit 3c25.

When using it in an actual application, please make a final decision after conducting sufficient preliminary verification.
Also, when using a PLL to generate the clock to be supplied to Nios II, there are restrictions on the generated clock due to the specifications of the PLL, so please refer to the minimum/maximum setting values described in the Device Handbook for each device.