I have a pihole that is connected to my home network but is NOT the DHCP server (not enabled) and at this time only gets a few DNS requests from clients that I have manually pointed to pihole.
For a number of weeks, I have had client computers take 30+ seconds to acquire a DHCP lease from my Linksys router. That problem was one reason I attempted to setup a replacement pi400 for pihole (see my previous help thread Problem with local hostname resolution)
I have now set up my original PI4 (pihole) device with new raspbian OS 32 bit and pihole setup. However, even though I have not customized it beyond bare minimum I still continue to have long dhcp lease acquire times on my home network.
In order to find a possible network problem, I disconnected EVERY client from my network and connected one computer directly to the linksys router (DHCP server) and received DHCP leases in a second or two. I then connected that one computer to my switch and the router/dhcp to the switch. Still normal dhcp acquistion times. I started adding one network device at a time and testing with normal results UNTIL I connected the raspberry pihole device to my switch. DHCP lease acquistion shot up to 30+ seconds. Disconnecting the pihole returned network dhcp lease acquition to normal.
One thing I can think of that perhaps could delay DHCP assignments would be your router DHCP server's duplicate address detection, which it would do to avoid that the same IP get's assigned to multiple NICs.
If you'd configured a static IP on your RPi 4, your router may try to assign that very IP address to a new client, and then DAD would kiick in and deny usage of that IP, since its already in use by your RPi 4.
To avoid that, you should either configure a DHCP lease reservation for your RPi's IP, or pick an IP address outside of your router's DHCP pool range.
You may also want to consider consulting other forums that specialise in DHCP/networking issues.
I'll keep looking for a non-pihole solution to my mystery and if I find the cause, I'll update this thread.
The pi-hole does have a static address on the PI and a DHCP reservation on the router/dhcp server. With the pi-hole disconnected from the network, a ping responds that the host is unreachable, so I don't think I have another device claiming the address. ADvanced IP Scanner also doesn't find any device at the pi-hole address (with it disconnected).
For the short term, I'm planning on trying to switch to the pi-hole DHCP server and disable the router dhcp service. Perhaps my older linksys router is not up to handling my growing IOT network.
I've also ordered a newer router that should arrive this week that I'll experiement with.
I'll also check out other networking forums that I'm aware of. A friend is also suggesting I try to use wireshark to look at the network but that does seem to have a steep learning curve!
I am determined to make pi-hole work for me as I really like its capabilities.
Thanks Bucking_Horn for your comments and assistance in this and my other previous help request.