Using open-source firmware won't get you much in the way of performance benefits if it's not using recent Linux kernels and well-maintained in-tree drivers.
And it was only recently that any routers started including CPUs that are powerful enough for high-speed packet processing. Prior to 802.11ac, basically everything was based off '90s-era single-core MIPS and the only hope of 100+Mbps throughput was to use hardware offloads that severely curtail the kinds of packet processing you can accomplish (and usually isn't supported by any open-source driver). Now we've got 1+GHz multi-core ARM processors in wireless router SoCs, but no mature drivers for them.
And it was only recently that any routers started including CPUs that are powerful enough for high-speed packet processing. Prior to 802.11ac, basically everything was based off '90s-era single-core MIPS and the only hope of 100+Mbps throughput was to use hardware offloads that severely curtail the kinds of packet processing you can accomplish (and usually isn't supported by any open-source driver). Now we've got 1+GHz multi-core ARM processors in wireless router SoCs, but no mature drivers for them.