Very slow sdcard reading on Pico/Pico2

Summary: I have been trying to playback music using Micropython but buffering results in choppy playback. Playback 'seems' to be about half the expected speed (Kim Wild singing Cambodia has a deep voice, LOL)

Note: Using Circuitpython (developed from Micropython?) using the same hardware and SD card plays music perfectly. So I decided to investigate.

Hardware: Cytron Maker Pi Pico pcb (includes SDCard), Waveshare I2S audio HAT, Sandisk Ultra 16Gb sd card , Pico 2W/Pico

Software: 

Pico 2: MicroPython v1.25.0-preview.393.gf1018ee5c on 2025-03-17; Raspberry Pi Pico 2 W with RP2350
Pico: MicroPython v1.24.1 on 2024-11-29; Raspberry Pi Pico with RP2040

I have tried as far back as MicroPython v1.16 on 2021-06-18; Raspberry Pi Pico with RP2040 - the timing results are very similar but there's no I2S so I can't even try playing music.

The following program ,that I wrote, opens a music WAV file and reads a number of 512 byte blocks from it, timing as it goes.

# SDCardSpeed.py
import sdcard, os
import time
from machine import Pin,SPI

print("machine freq",machine.freq())

##### SD Card setup ############
# SPI Pins are as per Cytron Maker Pi Pico pcb
#
spi=SPI(1,sck=Pin(10),mosi=Pin(11),miso=Pin(12))
cs= Pin(15,Pin.OUT)
sd = sdcard.SDCard(spi,cs)

os.mount(sd, '/sd')
#print(os.listdir('/sd'))

try:
    wav_samples_mv = memoryview(bytearray(512))
    
    with open("/sd/Cambodia.wav","rb") as wav:
        for count in range(4):
            start=time.ticks_us()
            num_read=wav.readinto(wav_samples_mv)
            print("File Read took ",time.ticks_diff(time.ticks_us(),start))
except Exception as e:
    print(f"EXCEPTION {e}")
    
spi.deinit()

In sdcard.py readinto() I wrapped the spi write_readinto() call to time the SPI transfer like this:-

        start=time.ticks_us()
        self.spi.write_readinto(mv, buf)
        print(f"sdcard readinto took {time.ticks_diff(time.ticks_us(),start)}")

After running SDCardSpeed.py with the default baudrate I got this output:-

machine freq 150000000
sdcard __init__ baudrate=1320000
sdcard readinto took 1537
sdcard readinto took 3898
sdcard readinto took 3882
sdcard readinto took 3883
sdcard readinto took 3888
sdcard readinto took 3882
sdcard readinto took 3892
File Read took  7915
sdcard readinto took 3893
File Read took  6882
sdcard readinto took 3892
File Read took  6879
sdcard readinto took 3892
File Read took  6906

It shows that the file reading, by the OS, is close to twice the time taken to read a 512 byte block from the card. But it gets worse...

Increasing the SD card baudrate to 20000000 by changing the sdcard instantiation with :-

sd = sdcard.SDCard(spi,cs,baudrate=20_000_000)

the SPI write_readinto() call took ~450us (great) BUT the file read took ~3900us. That's nearly 10x the time taken to read the card sectors.

machine freq 150000000
sdcard __init__ baudrate=20000000
sdcard readinto took 1538
sdcard readinto took 453
sdcard readinto took 441
sdcard readinto took 438
sdcard readinto took 438
sdcard readinto took 435
sdcard readinto took 444
File Read took  3905
sdcard readinto took 443
File Read took  3852
sdcard readinto took 441
File Read took  3845
sdcard readinto took 444
File Read took  3906

Therefore I conclude that slow SDcard handling on a Pico/Pico2 is NOT in sdcard.py but in the filesystem file handling. Or am I wrong?

This is a shame - my hobby project was going to involve sound recording and playback using an I2S microphone.

Who else needs to know this info?

Can anyone verify my results because right now I cant use micropython for this on Pico and Ciruitpython doesn't support I2S audio in (at this time)

Thanks

Start by running the attached SDCardSpeed.py on the listed Pico hardware and versions, then inspect sdcard.py, especially readinto() and its SPI transfer timing. Compare filesystem reads with direct block reads at both baudrates. Done means identifying the source of the extra read latency and documenting or fixing it with a reproducible test.