The max. supported I2C speed seems to be 400000.
In /boot/config.txt:
dtparam=i2c_arm=on,i2c_arm_baudrate=400000
Note: This could be caused by the P82B715 on the Pi01 with its max. 400kHz limit.
The Pico can only handle 100kHz.
If used as WIOpico the Zero should use the default I2C speed of 100kHz.