Someone already did that with their Raspberry Pi 4B, it's not about OS being incapable, mostly problem coming from "not configuring properly".
Some router hardware do have proprietary hardware acceleration which makes some difference between stock/OpenWrt (since OpenWrt has to stick with open source implementation), but for those with open source driver, it's the same, however stock firmware might already preset everything vendor believe it's good for performance (CPU scheduling, IRQ balance, etc....) but on OpenWrt you have to do it yourself.
And most home router stock firmware nowadays are just vendor modified OpenWrt as their OS (most of the time it's even older version), plus the driver I mentioned before, if there is no closed source implementation, I don't see any reason why using vanilla OpenWrt will cause performance impact on the same piece of hardware.