F5 GTM DNS - high number of requests

iRules are processed first in the line of the other services, so whereas there is always an impact, it will be negligible per connection compared to the other services processing those requests and that is a well-known and helpful approach to managing the issues you are seeing.

That said, the local system is still going to get all those requests, even if you're dropping them. A service like F5 Distributed Cloud DDoS Mitigation could scrub these brefore the requests hit your local services if the volume becomes too great for your equipment or pipes to manage.