I've always used custom firmware (openwrt on Asus 500gP/Linksys WRT54g, Tomato and then LEDE on Netgear WNR3500L) and I've been happier with them than stock stuff. For a long time the stock firmware (still true for the cheaper stuff) was really crappy in features compared to custom firmware missing basic stuff. These days the more premium stuff seems to have somewhat feature parity (I care about USB, IPv6, OpenVPN) so that's better especially since some of them seem to be forked from the open source projects (ex. AsusWRT).
One of the benefits of the stock firmware is taking advantage of hardware accelerated NAT and Wifi encryption (WPA) but when I installed LEDE a few months ago on the old Netgear WNR3500L it did give me the option to install a Wifi server that does use hardware acceleration so I can still get that nice high performance.
Note that all of those routers listed above are generally well supported by custom firmware because that's how I made my buying decision. My current main router (Asus AC66U) I left it with stock firmware as it has all the features I want and it's still getting updates often enough.
One of the benefits of the stock firmware is taking advantage of hardware accelerated NAT and Wifi encryption (WPA) but when I installed LEDE a few months ago on the old Netgear WNR3500L it did give me the option to install a Wifi server that does use hardware acceleration so I can still get that nice high performance.
Note that all of those routers listed above are generally well supported by custom firmware because that's how I made my buying decision. My current main router (Asus AC66U) I left it with stock firmware as it has all the features I want and it's still getting updates often enough.