This is a really low-power device. The warning basically says you that the load on the machine is on the order of what the device can offer. Looking at your debug log, it seems to barely keep up but if you add some more clients (or in general activity) on your network, it may simply be too much for this little device.
There isn't really anything you could do except replacing the device with a more powerful one. Maybe disabling the query database can bring you some reduction in workload (but probably it won't matter).
Thanks my friends for the follow up. Maybe in the prerequisite tell the version of raspberry pi that could be supported. I didn't even started to use that pihole as DNS filter and it's already too slow. I will probably use that raspberry pi as a decoration. I don't know what it could be use if it'S can't even run a pihole.
You should not read that as a general assessment of RPi B+ being inadequate for Pi-hole.
A first generation RPi B+ is a bit less performant as a Zero, which is quite capable of running Pi-hole. (Indeed, my own first RPi was a B+ as well, and it is still serving as a Pi-hole in a friend's home network without a speck to date.)
I think it's too early to retire your RPi B+, as your debug log does not indicate a high amount of DNS traffic:
[2024-01-30 06:15:20.431 474M] Imported 24 queries from the long-term database
[2024-01-30 06:15:20.432 474M] -> Total DNS queries: 24
[2024-01-30 06:15:20.433 474M] -> Cached DNS queries: 16
[2024-01-30 06:15:20.433 474M] -> Forwarded DNS queries: 8
[2024-01-30 06:15:20.433 474M] -> Blocked DNS queries: 0
[2024-01-30 06:15:20.434 474M] -> Unknown DNS queries: 0
[2024-01-30 06:15:20.434 474M] -> Unique domains: 4
[2024-01-30 06:15:20.434 474M] -> Unique clients: 1
If that is accurate, this would suggest that your observation is caused by something other than DNS requests overwhelming your Pi-hole.
I see ... any suggestion to solved that issue ? It's a raspberry pi that I only installed the newest image of raspberry PI OS (the one with debian 12). I wasn'T able to have an internet connexion throught the ethernet port even if I got 2 lights (solid orange, and flashing slowly green). SO I put an external network card that was automatically recognized.
I've made the update and install the pi hole ... nothing else has been install.
thanks again for your help
N.B. I didn't set that pihole as DNS server yet, so that'S why theres not high amount of trafic .. before using it I wanted to solved that issue. Since I think I have solved the issue from my pi hole from my pi zero, I will probably give ti to my sister in law who got huge problem with her autist daughter.
There are three versions of Pi OS. If you're not already, try re-imaging the card with the Lite version. This is a minimal build which does not have a desktop and does not have the extra packages (such as LibreOffice) installed. It boots to a terminal and you can still SSH in (if you're not using a keyboard and screen attached). This build is enough to install Pi-hole, which you will then access in the normal way via a web browser. It will help lessen the load if you've been using the fuller versions so far.
Edit to add – if you've already set up Pi-hole but want to try this, use Settings > Teleporter to take a backup which you can then restore later from the same place. That wil save you having to redo any lists and rules you've added.
Are you still seeing warnings about excessive load?
If not, it seems you just saw the result of some process (probably the update commands) causing a temporary excessive load. This is expected in a low-power device, like an almost 10 years old Raspberry Pi B+.