piolib: data transfers slower than expected

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#include "piolib.h"
#include "ws2812.pio.h"

#define bench_wrap_target 0
#define bench_wrap 0

static const uint16_t bench_program_instructions[] = {
            // .wrap_target
    0x6020, // out x, 32
            // .wrap
};

static const struct pio_program bench_program = {
    .instructions = bench_program_instructions,
    .length = 1,
    .origin = -1,
};

static inline pio_sm_config bench_program_get_default_config(uint offset) {
    pio_sm_config c = pio_get_default_sm_config();
    sm_config_set_wrap(&c, offset + bench_wrap_target, offset + bench_wrap);
    sm_config_set_sideset(&c, 1, false, false);
    return c;
}

static inline float bench_program_init(PIO pio, int sm, int offset, float freq) {
    pio_sm_config c = bench_program_get_default_config(offset);
    sm_config_set_out_shift(&c, false, true, 32);
    sm_config_set_fifo_join(&c, PIO_FIFO_JOIN_TX);
    float div = clock_get_hz(clk_sys) / freq;
    if(div < 1) div = 1;
    if(div > 65535) div = 65535;
    int div_int = (int)div;
    int div_frac = (int)((div - div_int) * 256);
    sm_config_set_clkdiv_int_frac(&c, div_int, div_frac);
    pio_sm_init(pio, sm, offset, &c);
    pio_sm_set_enabled(pio, sm, true);
    return clock_get_hz(clk_sys) / (div_int + div_frac / 256.);
}


double monotonic() {
    struct timespec tv;
    clock_gettime(CLOCK_MONOTONIC, &tv);
    return tv.tv_sec + tv.tv_nsec * 1e-9;
}

long databuf[1048576];

int main(int argc, const char **argv)
{
    float frequency = argc > 1 ? atof(argv[1]) : 10e6;
    size_t xfer_size = argc > 2 ? atoi(argv[2]) : 256;
    PIO pio;
    int sm;
    uint offset;

    pio = pio0;
    sm = pio_claim_unused_sm(pio, true);
    pio_sm_config_xfer(pio, sm, PIO_DIR_TO_SM, xfer_size, 1);

    offset = pio_add_program(pio, &bench_program);
    fprintf(stderr, "Loaded program at %d, using sm %d\n", offset, sm);

    float actual_frequency = bench_program_init(pio, sm, offset, frequency);
    fprintf(stderr, "Actual frequency %fMHz\n", actual_frequency/1e6);
    pio_sm_clear_fifos(pio, sm);

    double t0 = monotonic();
    size_t xfer = 0;
    do {
        pio_sm_xfer_data(pio, sm, PIO_DIR_TO_SM, sizeof(databuf), databuf);
        xfer += sizeof(databuf);
    } while(monotonic() - t0 < 1);
    double t1 = monotonic();
    double dt = t1 - t0;
    double rate = xfer / dt; // bytes per second
    fprintf(stderr, "%zu bytes in %.1fms (%.1fMiB/s)\n",
        xfer, dt*1e3, rate / 1048576);
    printf("{\"frequency\": %g, \"xfer_size\": %zd, \"rate\": %g}\n",
        actual_frequency, xfer_size, rate);
    return 0;
}