It's like almost every "cloud" works. This are just VMs with a lot of noise from your neighbors. That's why i always use bare metal. Control over whole CPU and cache lines can be game changer if you have cpu intensive applications.
If you are on shared tenancy dynos, and your current configuration is nearing maximum capacity, then adding dynos can reduce performance. Not per dyno, but across the entire platform.
Counter-intuitive but true.