Low power mode slow down my iPhone speed on Deco

Last week I had an odd experience. My iPhone SE 2020 and IPhone12 with IOS 14.6 always got a slow speed when connecting to Deco M5. Other devices, like Samsung Galaxy, as well Windows laptops all got the desired speed.

Yesterday, I came home, and surprisingly, it got a much higher speed. I did not change anything and could not figure out why. So I just let it go since I got the desired speed anyway. 

While this morning, the speed slows down again.  I started playing around with the settings on the iPhone again. Finally, I disabled the low power mode, then to my surprise, the speed recovered; Since the battery run out pretty fast on iPhone, I almost enabled the low power mode all the time which might explain why I always got the slow speed on iPhone.

Moreover, I tested again, enabling the low power mode and disabling the low power mode; then I have more findings after disabling the low power mode, the speed did not recover immediately until I turn off/ on the Wi-Fi again.

The speed without low power mode:

The speed with lower power mode:

I would like to see how many users are troubled by this issue as I do and also much curious what caused this issue.

Thank you.