On paper, the 16-inch MacBook Pro can drive up to two Pro Display XDR at 6k resolution without any issues. However, a number of 16-inch MacBook Pro users are complaining about performance issues with their machine after connecting it to an external monitor.
The dip in performance is happening irrespective of which monitor they connect to their MacBook Pro, be it a regular Full HD monitor or a high-end one like the Pro Display XDR. They complain that the CPU performance of the machine drops noticeably once it is connected to an external monitor.
The issue here is that when connected to an external monitor, the 16-inch MacBook Pro switches to the dedicated Radeon Pro 5300/5500M GPU. Even when idle, the GPU draws around 18-20W of power. Without the external monitor, the Radeon GPU draws around 5W of power. The additional 15W drain added by the dedicated GPU is deducted from the CPU’s thermal headroom which leads to it running at a notably lower clock speed of around 1.9GHz to 2.3GHz.
The additional power draw from the Radeon GPU even at idle also leads to the machine running hotter than usual for no apparent reason. One workaround to this problem is running the 16-inch MacBook Pro in clamshell mode as that leads to the external GPU only drawing about 5W of power. The issue does not seem to be present in the 15-inch MacBook Pro as it draws a reasonable 4-5W of power even when connected to an external display.
While the 16-inch MacBook Pro comes with the same Intel CPUs as the 15-inch MacBook Pro, it features AMD’s new 7nm Navi GPUs which are more powerful and power-efficient.
This likely seems like a driver issue in macOS Catalina which Apple will likely get around to fixing with an upcoming update of the OS. Otherwise, the crippled performance of the 16-inch MacBook Pro when connected to an external monitor would end up being a bummer for many power users looking to upgrade to it.[Via MR forums, 2]