I don’t have an external flash so I ran the test on a STM32 board with a Windbond memory chip. Although the test was in SPI mode, the same observation might appear in QSPI mode on nRF52840.
I used the “flash_speedtest” example sketch of Adafruit SPIFlash library. I noticed the clock was very strange. It was not steady and the duty cycle and speed varied drastically during the test. Below is the most “handsome” clock waveform and the speed was extremely low. Although the flash memory was recognized by the sketch, the erase test failed.
After changing the clock setting, the test passed and the clock waveform became much better.
flash.begin();
flashTransport.setClockSpeed(10000000, 10000000);
Would you change the speed and retry? nRF52840 supports up to 8MHz for SPI. I don’t know why the writing time looked more or less in the same range before and after changing the clock speed but at least the erase test passed. The library is beyond my capability to understand. Also the results from STM32 may not apply to nRF52840.