piHole cripples internet speed

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@Hawkrin

Will my network keep on working without a DNS server.

No, it will not.

@Hawkrin

Or should I "secure" it by entering for instance googles DNS as DHCP server 2 as backup?

You should not. Any DNS server available to clients is liable to be used at any time, resulting in DNS traffic bypassing Pi-hole.

Why should Pi-hole be my only DNS server?

Reduced Ad Blocking Capability The main reason you should Pi-hole as your only DNS server is that you will see increased performance in the blocking of ads. If you have two DNS servers (Pi-hole and something else), your network clients may not always query Pi-hole for name resolution. If a query happens to be answered from a non-Pi-hole DNS server, your block lists will not apply (since that DNS server doesn't know about them). All Queries Need To Go Through Pi-hole First Since other DNS serv…

One way to increase reliability (if you have problems keeping your Pi-hole running) is to add a second instance of Pi-hole on a different device or on a device that is already running 24/7 (a NAS, for example).

Then you would enter the IP's for each Pi-hole in the DHCP settings on your router. If either Pi-hole fails, the other continues serving the entire network with no network downtime.