delay(ms) only delays for about 96% of the requested time on Pico and Nano RP2040. micros() seems to run at the correct rate.

The following program demonstrates the problem. loop() has a delay(1000) but executes in about 960 microseconds.

uint32_t start;
void setup() {
 Serial.begin(9600);
 while(!Serial) {}
 start = micros(); 
}
uint32_t count = 0;
void loop() {
 Serial.print(count++); 
 Serial.print(' '); 
 Serial.println((micros() - start)/1000000.0);
 delay(1000);
}

Here is output. The first column is loop count and the second column is time in seconds. The second column should be >= to the first column.

0 0.00
1 0.96
2 1.92
3 2.88
4 3.84
5 4.80
6 5.76
7 6.72
8 7.68
9 8.64
10 9.60
...
93 89.28
94 90.24
95 91.20
96 92.16
97 93.12
98 94.08
99 95.04
100 96.00

I suspect the problem is due the RP2040 having a CPU rate of 125 MHz but in:

D:\mbed-os-rp2040_pr\targets\TARGET_RASPBERRYPI\TARGET_RP2040\TARGET_NANO_RP2040_CONNECT\board.c

uint32_t SystemCoreClock = 120000000;

I think micros() returns the RP2040 hardware microsecond clock but delay() uses an mbed wait based on an incorrect SystemCoreClock.