Forgive me for the links...just google them and google will correct them for you. It is stupid thing that I can only have 2 links =/ ) [edit - I can now post full links and update the post]
Pandora box (check www.github.com/yjhjstz/pandorabox/blob/master/openwrt_version ) is based on a version of Sep 2013 (v12) (check dev.archive.openwrt.org/changeset/38121.html )
At this point it has MAJOR vulnerabilities. If you put the router on the Internet, it will be hacked pretty quickly.
As far as I know, the original mi wifi mini firmware is also based on Openwrt v12. So pandorabox can use the original drivers developed and optimized by MI for the router. That is why it is so good.
As far as I know, when Openwrt switched from the Linux kernel v3 to v4 the driver for the MT7620A had to be rewritten.
Looking at the forum and on Google, the MT7620A has a very bad track of records. People report slow performances on a wIfi and speed dramatically dropping when the USB is being used. See for example: Worse 2.4GHz and best 5GHz wireless performance after flashing LEDE
There has been a lot of work on that driver but it seems that it is still not in a good shape.
I remember also reading that once you install the latest version, and you try to go back to the original firmware, the performances are never going to be the same. Maybe there are tuning values stored somewhere that gets overridden?
I also saw this githubcom/chenhaiq/mt7610u_wifi_sta_v3002_dpo_20130916
It talks about USB fixes. I am not sure if anybody looked into it.
This seems going in the right direction: MT7620A Wifi very slow. Is there any driver issue?
And as far as I know, this is the tread with more updated info (and some drama): NEXX 3020 (MT7620) Wi-Fi issues
Has anybody tried to install the files here: https://github.com/openwrt/openwrt/pull/626#issuecomment-360844957 ?
Anyway, I would also like to install the latest v18 but I would also love to have the stability and the performance of the original firmware.
If somebody has questions or updates, please let me know.