Matter seems to tax memory a lot on pre-C8 Pros. You'll see you have less available memory after starting it. Which makes any 'normal' gradual decline in free memory more impactful on hub operations. I imagine Matter resource usage played a role in the timing of the release of the C8 Pro.
I'd also consider Matter devices to be beta. My only ones are Third Reality. The first firmware version was OK, but quirky. They seem to have settled and are working no differently than any other device. If you are using Matter devices over WiFi you are at the mercy of your WiFi and network performance too.
If you are using Matter over Thread then that device does need a border router in place. If you are using Matter over WiFi then you do not need to keep the commissioning device/service in place.
I think there is a limit of 5 controllers for a device. I've not had an issue with 4. Any performance issues would likely be device related.
I added my first matter device to C7. A Hue hub dont have other. Went smootly, no problems. Nice job. Altought it cannot be used for anything at the moment. Nothing in it. Uninstalled straight away.
I have two EVE outlets now on my C7. Very straightforward and they seem to work really well. They are also on HomeKit and Google Home and work well across all of them. Will be nice in the future to just get devices that work across platforms.