Second serialPort slows down serial communication in Unity

Hi everyone, my project integrates a depth sensor and an IMU (connected to Arduino). Whenever I connect the depth sensor to the USB port, the speed at which Unity reads from Arduino becomes extremely slow. (Performance of sensor isn’t changed, however) I thought it’s because I’m using a USB splitter which cause congested hub, but when I switch to the other computer with two separate USB hubs (left and right sides of computer), it seems improved but still cannot compare to the speed when sensor is not connected.
Any comments or tips on how to solve it would be very appreciated!

[BUMP]