There is no widely supported "custom" firmware (like OpenWrt or DD-WRT) for this specific model due to its hardware constraints (16Mb Flash, 64Mb RAM). The "better" firmware is always the most recent version provided by Wavlink.
If you have a high-speed fiber connection (over 100Mbps), the hardware itself is your bottleneck. In this case, the "better" solution isn't firmware—it's an upgrade to a dual-band router. Summary: The Path to Better Performance wlwn523n2 firmware better