[teruakohatu]

With 'normal' DNS, UDP with the default and TCP is used if the packet size becomes too large. There are other TCP-only variants such as DoT (DNS over TLS) and DoH (DNS over HTTPS).

I don't think the performance would matter much with some basic caching (or even just OS-level caching), but there is limited memory in an ESP so maybe that is it. I have never noticed issues with DoT and DoH which are theoretically much heavier protocols.