Not true at all, if by 10MB/sec you mean 80Mbps.
On my Pi 2 with an Edimax EW-7811UTC I have measured throughput of as high as 130Mbps (yes faster than Ethernet) on 5Ghz in the same room as the base station. (This adaptor is much slower on 2.4Ghz though as it does not support MIMO)
With a TP-Link TL-WN823N I can achieve 115Mbps in the same room on 2.4Ghz. (This adaptor does not support 5Ghz but does support 2x MIMO)
These throughputs were all measured using iperf to an Ethernet connected Mac.
Here are some benchmarks I did on OSMC RC2 taken from a base station in a downstairs living room through the ceiling to an upstairs bedroom:
Wifi benchmark to upstairs bedroom Pi 2 from Superhub 2. (AC)
Jellyfish - highest bitrate that will play without pauses with no buffering enabled.
iPerf - 1MB window size - range from 4 tests in a row. Kodi running but idle.
Tenda W311M (rt2x00)
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Jellyfish 10Mbps
Iperf receive 20.2Mbps - 26.1Mbps
Iperf send 14.0Mbps - 16.7Mbps
D-Link DWA-131EU (8192cu)
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Jellyfish 25Mbps
Iperf receive 55.0Mbps - 62.9Mbps
Iperf send 13.5Mbps - 18.3Mbps
TP-Link TL-WN823N (8192cu)
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Jellyfish 40Mbps
Iperf receive 97.8Mbps - 99.5Mbps
Iperf send 75.5Mbps - 78.4Mbps
Edimax EW-7811UTC - 2.4Ghz (rtl8812au)
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Jellyfish 30Mbps
Iperf receive 42.5Mbps - 49.6Mbps
Iperf send 37.8Mbps - 40.5Mbps
Edimax EW-7811UTC - 5.0Ghz (rtl8812au)
--------------------------
Jellyfish Failed to connect to SMB share
Iperf receive 79.2Mbps - 92Mbps
Iperf send 86.7Mbps - 87.7Mbps
The problem with your test is that the USB connected hard drive and file system provides an additional bottleneck. You are not testing the network performance, you’re testing a combination of network performance and external hard drive write performance. And both Wifi adaptor and hard drive are connected on the same USB bus. The USB bus on the Pi is relatively crude and isn’t what you would call high performance, so two devices active at once involved in the transfer is going to slow things down a lot.
Then you have the actual write performance of the drive, whose effects can’t be ignored. If you’re really looking for the best performing wifi adaptor, use iperf to measure only the wireless performance.
As you can see from my results, the speeds you can get are highly dependant on the individual adaptor, and also the base stations capabilities and how those mesh with your chosen adaptor.