DIY WiFi: connectivity issues with CyanogenMod 12.1

I have a home network built around an ancient computer that I've tued into a router, running Debian Wheezy. The wireless part is based on a TP-LINK TL-WN851ND PCI card on the hardware side, and uses hostapd on the software side.

Now, I have recently obtained a new phone, on which I am running CyanogenMod 12.1 (equivalent to Android 5.1). This device can connect to my wireless network, but most network operations (e.g. downloading things) result in timeouts, requiring retries. Fetching emails over IMAP is particularly terrible, almost invariably resulting in timeout exceptions. If a connection succeeds, it's fast, but getting the connection to succeed is problematic. It is only on this network that I'm getting these problems. Other devices, in particular a laptop running Gentoo Linux and an old phone running stock Android 4.x, have no problems on this network. I had a phone running CyanogenMod 11 (now broken) that occasionally had trouble connecting but usually worked, but the new one usually doesn't. This leads me to believe the problem is with my wireless network rather than with the phone.

One thing I have discovered just now is that pinging the gateway and any wired hosts within the LAN using the phone always gives 0% packet loss, whereas pinging exteal hosts or bridged VPN hosts averages about 50% packet loss. Pinging the same exteal hosts from wired computers in the LAN does give 0% packet loss. I've used bare IP addresses to test this, so DNS isn't the problem.

hostapd configuration:

debug=2    
interface=wlan0
bridge=br0
driver=nl80211
country_code=NL
ieee80211d=1    
logger_syslog=-1
logger_syslog_level=0
logger_stdout=-1
logger_stdout_level=0    
ht_capab=[HT40-][SHORT-GI-40][DSSS_CCK-40][MAX-AMSDU-3839]
auth_algs=1
hw_mode=g
channel=10
ieee80211n=1
wmm_enabled=1
ssid=redacted
wpa=2
wpa_psk_file=/etc/hostapd/wpa_psk
ignore_broadcast_ssid=0    
wpa_group_rekey=3600

I've tried various ht_capab stanzas -- including omitting it entirely -- to no avail. I've also tried configuring the network using a static IP address in CyanogenMod, but this too does not work. Using a WiFi analyser app reveals that this network is the only one on its channel, and provides the strongest signal of all nearby access points. The syslog on the router does not reveal anything interesting apart from periodic re-authentication.

Thanks in advance.

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