Rate mult is too low

I connected the PX4’s TELEM2 serial port with the onboard computer and tried to read the PX4’s IMU data via Mavlink.But it was found that the rate mult was too low, resulting in the frequency of setting 1000hz and the actual frequency was only 50hz, I want to know what is the reason for this, and how to solve it, thank you!
My setup instructions is:mavlink stream -d /dev/ttyS3 -s HIGHRES_IMU - r 1000.