16 bit IQ data convert to 2 bit IQ data

I have a large 16 bit I and 16 bit Q datafile need convert to 2 bit I
and 2 bit Q datafile.

I just use mask 0xC000 to keep first MSB 2 bit for each I and Q. But I
get performance issue.

In the 16 bit datafile I get about 42 dBHz C/N0, but in the 2 bit IQ I
only get 32 dBHz C/N0.

Is there any method I can improve the (2bit IQ datafile) performance?