I'm running Openwrt CC on orange pi zero, I use it as a router with its built-in wifi. When I transfer files from my computer to the opiz, I get 1MBps of transfer speed, is this normal ? Is it possible to increase the transfer speed by using an external USB wifi ? On my android phone it gets even worse, even loading pictures from the opiz through ES File Manager lags a lot.

Yes, this is more or less normal. You could not choose worse hardware for this job  

 

USB wifi will help getting to normal speed, yes. but you have to pick one carefully.

Hillz the zero wi-fi is slow, but that is very slow,  check the channel you are using make sure to use the cleanest one.

 the stock antenna are crap replace.

 at 30 feet with two walls under heavy load I see 3~4 meg transfer, with small text files I see much faster speeds.

 

 

If you use a usb wi-fi make sure you have a GOOD 5 v / 2~3 amp PSU

Some backgroundinformation about wifi on sbc:

Search for wifi opi zero on forum:

 

The wifi on the opi zero isn't made for ap mode. The driversituation is somehow questionable. 

 

Edit: igor was faster...