Lot of client, RPi4 better than 3

I'm looking for advices, I have around 50 "devices" (physical or VMs) who run on my network. and a pool of 100 IPs

Pi-hole is managing DHCP & DNS

I'm running all on a Rpi 3 B+ on top of DietPi

I had some slowness recently with devices don't getting IP from DHCP ...
I can also see that the CPU is starting to become hot and DietPi gave me warning it when I went to reboot the device

Will it help to move my setup to a new Rpi 4 or not really?
Is the load an issue for Pi-hole or the device?

I have around as many devices as you when I count IoT devices, tablet, phone, computers. I use a 3B+ for pi-hole/dhcp as well. My block list is ~1MM. It also runs unbound, a reverse proxy, inadyn, node for some cron'd light control, etc. It stays in the 36-37C range most of the time with load being extremely low most of the time. I did purchase a small case and a fan for it though. I have used that from the beginning. You might need to inspect various logs to see what might be using resources and such but a 3B+ would seem like enough, IMO.

In general, Pi-hole would run just fine even on a low resource device like an RPi Zero.

Since you mention devices not being issued a new DHCP lease specifically:
Do some of your devices use MAC address randomisation?

In an attempt to keep IP addresses fixed for a specific MAC, Pi-hole's dnsmasq may run dry on IP addresses if it is confronted with too many different MACs.

In that case, one solution would be to increase your DHCP pool size, but ultimately, the better approach would be to prevent a client from randomising its MAC when it is connected to your home network (where your Pi-hole resides). It would depend on the client device and its OS how you'd achieve that.

Thanks, it's seams "private address" is activated on all the iOS devices ...maybe an update re-activated it.
let see if disable it can improve the situation.
my Pi is at 57°C ... and it's new I updated based OS but didn't touch the hardware for year.

I agree that running Pi-hole as DNS and DHCP server shouldn't lead to such significant load, even with this amount of clients. If disabling "private address" on iOS doesn't help with the situation, you may check htop for which processes do either currently have a high CPU usage or which have an overall high CPU time, to hopefully detect the culprit. And as a general healthy check, kernel and system logs are good to have a look at:

dmesg | tail -20
journalctl | tail -50

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