DNS issues have, more often than not, caused networking slow downs for me. Running a recursive DNS server on a home network is quite a bit slower than using a public DNS server on a high speed network; the slowdown with a local cache is less, but still there. Just directly using 8.8.8.8/8.8.4.4 or 9.9.9.9 or 1.1.1.1 or 4.2.2.1 is best (faster, more reliable) in my experience: Fewer moving parts. There are significant privacy and security issues with using DDNS addresses which can be resolved by public DNS servers.
For the record, I have written a DNS server from scratch. Three times, actually (try 1, which is still the authoritative DNS server I use for my domains, try 2 which is a tiny caching DNS server, and try 3 -- which, yes, reuses code from try 2 -- is a very flexible DNS server which uses Lua for configuration).
Your external DNS server is quicker than a local cache? My local cache adds less than 1 millisecond latency to an uncached lookup, and answers queries for all LAN computers in less than 1 millisecond as well.
My DNS server is pretty fast under ideal circumstances (under 0.07ms per reply using 2000 era hardware as per https://maradns.samiam.org/speed.comparison.html ). I’m sure you’re not getting 1ms in less-than-ideal circumstances (router overloaded and dropping packets, which sometimes happens on my home network), where that extra DNS server starts to really slow things down.
Ya my network never drops packets, at least for congestion reasons. Seems like congestion will affect external servers at least as much as internal ones, though.
(Access to my DNS server is not routed on my LAN, it's a flat network.)
For the record, I have written a DNS server from scratch. Three times, actually (try 1, which is still the authoritative DNS server I use for my domains, try 2 which is a tiny caching DNS server, and try 3 -- which, yes, reuses code from try 2 -- is a very flexible DNS server which uses Lua for configuration).