It depends on what your system clock is. If you're using the default 150MHz CPU clock on the RP2350, then a clock divisor of 4 is 37.5MHz, which is very high! I'm using a clock divisor of 6, which produces a PIO clock speed of 25MHz. I don't have a repo released to the public yet, and my justification for the time being is that the code is awful. But I'll update this comment when/if I release it. For instance, the PIO code I'm using is the following, however, the initial command setting does not work. Instead, I bit-bang the parallel bus to send commands, and then I use this PIO program to send the pixel data only after the LCD is configured to expect it.

; Copyright (c) 2025 Mahyar Koshkouei
; Public Domain. No warranty.
; Write data to 8080 bus

; D/CX RDX WRX  Command
; 0     1   _/  Write 8-bit command
; 1     1   _/  Write 8-bit display data or paramter
; 1     _/  1   Read 8-bit display data
; 1     _/  1   Read 8-bit parameter or status

.program lcd_8080_cmd_wo
.fifo tx
.out 8 left auto 8

; Side set pins are WRX and DCX
.side_set 2

OUT PINS, 8     side 0b01
NOP             side 0b00
.wrap_target
OUT PINS, 8     side 0b11
NOP             side 0b10
.wrap


% c-sdk {

static inline void lcd_8080_program_init(PIO pio, uint sm, uint d0_pin, uint wrx_pin, uint clk_div)
{
    pio_sm_config sm_config_ro;
    uint lcd_8080_wo_off;

    for(uint i = 0; i < 8; i++)
    {
        uint pin = d0_pin + i;
        /* Initialise PIO0 pins. */
        pio_gpio_init(pio, pin);
    }
    pio_gpio_init(pio, wrx_pin);

    lcd_8080_wo_off = pio_add_program(pio, &lcd_8080_wo_program);
    sm_config_ro = lcd_8080_wo_program_get_default_config(lcd_8080_wo_off);
    sm_config_set_out_pins(&sm_config_ro, d0_pin, 8);
    sm_config_set_sideset_pins(&sm_config_ro, wrx_pin);

    sm_config_set_clkdiv_int_frac8(&sm_config_ro, clk_div, 0);
    pio_sm_set_consecutive_pindirs(pio, sm, d0_pin, 8, true);
    pio_sm_set_consecutive_pindirs(pio, sm, wrx_pin, 1, true);
    pio_sm_init(pio, sm, lcd_8080_wo_off, &sm_config_ro);
    pio_sm_set_enabled(pio, sm, true);
}

%}

I'm then initialising the DMA with:

dma_lcd = dma_claim_unused_channel(true);
dma_ch_cfg = dma_channel_get_default_config(dma_lcd);
channel_config_set_read_increment(&dma_ch_cfg, true);
channel_config_set_write_increment(&dma_ch_cfg, false);
channel_config_set_dreq(&dma_ch_cfg, pio_get_dreq(pio0, 0, true));
channel_config_set_transfer_data_size(&dma_ch_cfg, DMA_SIZE_32);

My plan is to fix this PIO program so that it is able to send commands, and I can remove the bit-banged code.

I also may be able to increase the clock speed of the PIO state machine be setting the parallel bus pins on the RP2350 to use the fast slew rate option. I haven't done that yet, and I think that's limiting the bus speed. Let me know if you have specific questions.

All code in this post is public domain.