With nothing on the GPIO pins (including no fan shim) the I2C works normal.
sudo i2cdetect -y 1 completes all addresses near instantly.
Mount the Fan Shim, mechanical only - no software: I2C fails.
sudo i2cdetect -y 1 takes over a second per address
How to prevent the Fan Shim from interfering with I2C?
Official RPi4 power supply
Buster 4.19.75-v7l+ #1270