Wireless network slows down with zyd0

[Rouan Lucas van Ryn]

I have been experiencing slow network speeds with my FreeBSD (11.0)
system on my wireless LAN.

My wireless card is a USB ZyDAS card:
zyd0: <ZyDAS USB2.0 WLAN, rev 2.00/48.10, addr 2> on usbus3

When I start networking, wireless speeds seem OK (~700KB/s when scp'ing
a file to my laptop on the WLAN), but after around 10 seconds of network
activity drop off to become very slow (~70KB/s). Speeds stay slow until
networking is restarted and then pick up again momentarily. Tests with
iperf yield similar results.

I have found that the wireless connection is being renegotiated at lower
and lower speeds:
wlan0: [0c:d6:bd:1a:19:24] AMRR: current rate 72, txcnt=11, retrycnt=17
wlan0: [0c:d6:bd:1a:19:24] AMRR decreasing rate 48 (txcnt=11 retrycnt=17)
wlan0: [0c:d6:bd:1a:19:24] AMRR: current rate 48, txcnt=11, retrycnt=14
wlan0: [0c:d6:bd:1a:19:24] AMRR decreasing rate 36 (txcnt=11 retrycnt=14)
wlan0: [0c:d6:bd:1a:19:24] AMRR: current rate 36, txcnt=11, retrycnt=13
wlan0: [0c:d6:bd:1a:19:24] AMRR decreasing rate 24 (txcnt=11 retrycnt=13)
wlan0: [0c:d6:bd:1a:19:24] AMRR: current rate 24, txcnt=11, retrycnt=11
wlan0: [0c:d6:bd:1a:19:24] AMRR decreasing rate 22 (txcnt=11 retrycnt=11)
wlan0: [0c:d6:bd:1a:19:24] AMRR: current rate 22, txcnt=11, retrycnt=13
wlan0: [0c:d6:bd:1a:19:24] AMRR decreasing rate 18 (txcnt=11 retrycnt=13)
wlan0: [0c:d6:bd:1a:19:24] AMRR: current rate 18, txcnt=11, retrycnt=11
wlan0: [0c:d6:bd:1a:19:24] AMRR decreasing rate 12 (txcnt=11 retrycnt=11)
wlan0: [0c:d6:bd:1a:19:24] AMRR: current rate 12, txcnt=11, retrycnt=12
wlan0: [0c:d6:bd:1a:19:24] AMRR decreasing rate 11 (txcnt=11 retrycnt=12)
wlan0: [0c:d6:bd:1a:19:24] AMRR: current rate 11, txcnt=11, retrycnt=11
wlan0: [0c:d6:bd:1a:19:24] AMRR decreasing rate 4 (txcnt=11 retrycnt=11)
wlan0: [0c:d6:bd:1a:19:24] AMRR: current rate 4, txcnt=11, retrycnt=11
wlan0: [0c:d6:bd:1a:19:24] AMRR decreasing rate 2 (txcnt=11 retrycnt=11)
wlan0: [0c:d6:bd:1a:19:24] AMRR: current rate 2, txcnt=11, retrycnt=11

The same wireless card works well (no slowdown) when connected to a
different computer (not FreeBSD) in the same room, leading me to suspect
that this isn't a hardware or signal strength issue.

Netstat shows that packets are being retransmitted:
tcp:
        109179 packets sent
                108551 data packets (110570192 bytes)
                575 data packets (534156 bytes) retransmitted
                6 data packets unnecessarily retransmitted
				...

Relevant parts of ifconfig:
zyd0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 2290
	ether 00:25:xx:xx:xx:xx
	nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
	media: IEEE 802.11 Wireless Ethernet autoselect mode 11g
	status: associated
wlan0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
	ether 00:25:xx:xx:xx:xx
	inet 192.168.1.6 netmask 0xffffff00 broadcast 192.168.1.255 
	nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
	media: IEEE 802.11 Wireless Ethernet DS/1Mbps mode 11g
	status: associated
	ssid xxxx channel 1 (2412 MHz 11g) bssid 0c:d6:bd:1a:19:24
	regdomain APAC country NZ authmode WPA2/802.11i privacy ON
	deftxkey UNDEF TKIP 2:128-bit txpower 30 bmiss 7 scanvalid 60 bgscan
	bgscanintvl 300 bgscanidle 250 roam:rssi 7 roam:rate 5 protmode CTS
	roaming MANUAL

Note in particular the line:
	media: IEEE 802.11 Wireless Ethernet DS/1Mbps mode 11g

ifconfig wlan0 scan gives me:
SSID/MESH ID    BSSID              CHAN RATE   S:N     INT CAPS
SPARK-DFWFMP    0c:d6:bd:1a:19:24    1   54M -58:-95  100 EP   RSN HTCAP WPS WPA
(among other networks)

-- 
Thanks,
Rouan