I did some tests with the NEO Air in the recent days, borrowing the antenna from an OPi Zero. Since I experienced some serious lag while typing over ssh, one of the first moves was putting the thing line of sight to the router. This helped a little, but not much.
Now I have been playing with iperf and several different setups. I have a second antenna that came with an OPi One. It's the same model that comes with the Zero, but seemed to double the performance. I saw a jump from 10 MBit/s to 20 MBit/s.
Then I tried putting an old harddisk behind the antenna as as some sort of makeshift reflector (varying the distance as well). In one case it seemed to boost the signal, iperf went from something around 20 MBit/s to 30 MBit/s. Also, placing the HDD between the antenna and the router significantly decreased the signal.
Orienting the antenna differently also had an effect. At one point performance even went down to 1 MBit/s. That's when I realised the u.FL-connector was no longer properly connected.
So here is my takeaway: While they might do the job, OPi's stock antennas may vary in quality. I guess there is no quality check at the end of the assembly line. Also, it is very easy to screw up things. The consumer mindset -- plug it in in and expect it to work flawlessly -- is certainly not the right approach here.
The good news is: To a certain degree you can improve things by optimizing the antenna. I see a bright future for tinfoil reflectors and cantennas. . .
Boring details: There are more than 50 Wifis in the 2,4 GHz band visible at my place, so none of this qualifies as a science experiment. When using iperf, distance to the router was apr. 4 metres through air and a brick wall. Above ratios came from iperf's default mode. Reverse operation (iperf3 -R) makes roughly a +10 MBit/s difference.