Such as the title, in the Android 6.0 system, the speed is limited to 2MB or so, in other wireless router at least 5MB above the speed.
Previously thought it was a problem after the mobile phone system update, until a few friends with different models, there have been such problems. Mobile phone upgrade to Android 7.0 still have the same problem, can only say ros compatibility there is a big problem.

What kind of phone, specifically? How are you testing the speed? Such low speed could also indicate some issue with hardware or phone drivers.

I have Android 6.0.1 and have not experienced any issues with the RouterOS APs. How the wireless interface is configured, what security profile (if any) is used? Maybe - what phone specifically you are using?

Using the mi, oppo, vivo these three brands, as long as the system is above the Android 6.0 there is such a problem is to use speedtest test.
Brush off third parties such as CM13, CM14, the problem remains.
Mobile phone hardware MTK Qualcomm, ros is the default configuration.