Increasing events of delays with zigbee devices

Hey there
I’ve been building my system for three years now. I am using a rpi 4 (8gb ram + SSD) with a Slaesh’s CC2652RB stick. Apart from HA plus a couple of integrations I don’t run anything else on that system. Pretty sleek I’d say.
I never experienced problems in connectivity or response with my zigbee devices. Every now and then I integrate a new device, adding up to appr. 60 or 70 now. After having integrated the last devices I noticed some sporadic delays when switching them on. Not only manually but also via scheduled switching. One example is my bathroom light, which is triggered by a motion detector. And this one is to switch on a wall switch with two channels. And sometimes one channel is taking up to 10 seconds or more to turn on whereas the other channel works without delay. Funny thing is, that this applies to new devices but also to devices that have been in use since the very beginning.
Long story short: obviously there is a bottleneck somewhere. Is it the zigbee coordinator? However I read that they can work with up to 120 devices. Or is it the system itself? Is my rpi reaching its limit?
Grateful for any advice.

Rico

Could be that you have installed some very chatty device.

So what would I have to do to pinpoint the faulty device?
Try to reproduce the faulty behaviour and check the mqtt logs afterwards?
So for what kind of entries would I have to look for?