I am not sure why I can't reach target sampling rate by the following code:

import time

sdr.sample_rate = 2.048e6 
sample_num = 1024 * 128
iq = []
n=100   #n cycles
tic = time.time()

for _ in range(1,n+1):
    iq.extend(sdr.read_samples(sample_num))

toc = time.time()
dt = toc-tic
sr = len(iq)/dt
print(f'sample rate setting: {sdr.sample_rate}, actual sample rate = {sr}')

result:
sample rate setting: 2048000.0, actual sample rate = 1888409.6998254296

I could only reach target fro samplle_rate<=300KHz

Is there any way I could omptimize my code?
Should I jump to stream(), which read sample in a async way?