The Dashboard is quite a complex beast but I know that many people have really complex Dashboards. The main limitation is around the use of the standard chart node where you really need to ensure that you keep the number of displayed points under control.
There have been a few discussions about this over time but the overall indication seems to be that you really can push MQTT a long way. However, it does somewhat depend on what broker you are using and what else is running on the server device. If you are using Mosquitto as your broker, it is compact and highly performant, not sure what msg rate it will run up to but it is pretty high even on a Raspberry Pi.
As always, make sure that you aren't overloading your server device. Especially with something like a Pi. Don't run Linux desktop for example if you don't absolutely have to - and if you do, expect greatly reduced performance. If you are running a desktop on the server, don't use the server to browse the Dashboard because that gives you a triple whammy.