5935 - MachXO2: When using the XO2/XO3 device as a slave, is it OK to use an I2C bus speed that is lower than what's specified in Diamond GUI? (for example, use 234KHz SCL clock speed provided by an external master instead of 400KHz)
As long as the clock speed for the wishbone bus is at least 7.5x higher than the I2C (SCL) bus speed, there shouldn't be any issue.
To further check this, it is recommend to perform Gate-Level Timing Simulation.