Why the screen refresh so slow with the background img(bin)?

Description

What MCU/Processor/Board and compiler are you using?

STM32f407

What do you want to achieve?

What have you tried so far?

The led winking 50ms using the code:
lv_timer_t * task_1 = lv_timer_create(u_task_1, 50, ui);
void u_task_1(lv_timer_t * task_1)
{ …
lv_led_toggle(ui->screen_led_1);
}
It runs OK . But the screen refresh frequency will be slow if display the background img which is a bin file using fatfs load. Besides,the refresh frequency is OK if the img is C array be in MCU.
How to solve the problem? thanks a lot.

Code to reproduce

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