I put this setting in ages ago on my FF profile and haven't seen DNS lag.

My biggest DNS lag was before I used PiHole and was relying on my router, which upstream to 8.8.8.8. I've just assumed that little thing was overloaded or that Comcast was just having a "hiccup".

I see glacial DNS resolution regularly when hitting the AWS authorization page with DoH disabled on my company's VPN. Resolves instantly in Chrome.

This is almost certainly a fragmentation issue caused by lower MTU and broken path MTU on the VPN. Drop the system to 1280 to troubleshoot, if things work immediately there's the culprit, raise it up til it doesn't or don't, I keep my VPN's at 1280.

EDIT: I do not know why its an issue with firefox and not chrome, it's likely QUIC fucking up since it cant fragment and needs to fall back to TCP, chrome is probably error handling this better... dropping the MTU that low will make the fallback explicit: https://blog.apnic.net/2019/03/04/a-quick-look-at-quic/

EDIT2: Could also try disabling QUIC, instructions here: https://developers.cloudflare.com/cloudflare-one/policies/ga...