I experienced this issue on gentoo. I eventually figured out that it was caused by power management for the wireless device. Check whether your kernel was built with CONFIG_CFG80211_DEFAULT_PS. I can transfer data all night without the speed drop if I disable this option.
As an alternative to rebuilding your kernel, there may be a boot option to disable power management, or you can try iwconfig wlan0 power off, but I get "Operation not supported" with iwconfig and I'm not sure why.
I experienced this issue on gentoo. I eventually figured out that it was caused by power management for the wireless device. Check whether your kernel was built with CONFIG_ CFG80211_ DEFAULT_ PS. I can transfer data all night without the speed drop if I disable this option.
As an alternative to rebuilding your kernel, there may be a boot option to disable power management, or you can try iwconfig wlan0 power off, but I get "Operation not supported" with iwconfig and I'm not sure why.