Yun SD datalogging I2C with 4 MPU6050 via Mux, very slow

Hello,
I have connected four MPU6050 to the Yun via a Multiplexer and would like to log the measurements on the SD-Card.
Apparently, it runs very slow at about 1Hz (already without displaying the data on the serial monitor).
Target would be around 50Hz for each sensor.
I looked through several posts, but could not figure out yet how to get it faster.
Any help is really appreciated. Thank you!