Wi-Fi: too many "Tx excessive retries" and "Invalid misc"

I have an old laptop running LXLE (an Ubuntu-based distro) with a Broadcom BCM4311 Wi-Fi card, and although Wi-Fi connection works, data transfer speed is at roughly half of what it should be when connected to an 802.11g wireless network. I suspect it is because iwconfig reports large numbers of "Tx excessive retries" and "Invalid misc" -- tens of both immediately after boot, thousands after an hour of relatively casual use (email, connection to a remote machine via RDP). The laptop is located near the Wi-Fi AP (ca. 2 m) and iwconfig almost always reports 54 Mb/s bitrate. In spite of this, copying a file from an otherwise idle server on the local network runs at ca. 2 MB/s at best. As for the driver, I am using b43. broadcom-wl also works; however, I see no improvement over b43. IPv6 is disabled, MTU is set to 1492 (determined using http://muzso.hu/2009/05/17/how-to-determine-the-proper-mtu-size-with-icmp-pings ; this value most probably is due to the VDSL internet connection). I also tried to change the network stack configuration ( https://www.cyberciti.biz/faq/linux-tcp-tuning/ ), but that made no difference either. The Wi-Fi network is operating on channel 5, while the other two networks are both on channel 11 (a scenario that is quite rare these days, I know...). There are no nearby microwave ovens or other equipment capable of causing significant interference. I never experience such problems in the same LAN (with the same AP) with an even older LXLE laptop containing an Intel Wi-Fi card. This older laptop can easily transfer files from the aforementioned LAN server at 5.5 MB/s over 802.11g, and iwconfig reports zero "Tx excessive retries" and "Invalid misc" there. Is -- perhaps I should write "was" when referencing such ancient hardware -- BCM4311 simply a pain to run under linux without issues? What else can I try to improve the Wi-Fi performance?