Unfortunately it’s the GPU that causes the issue, not the CPU.
Whatever bus video runs over is wired through the dedicated GPU, so integrated is not an option with external monitors connected. That by itself would be fine, except for whatever reason, driving monitors with mismatched resolutions maxes the memory clock on the 5300M and 5500M. This causes a persistent 20W power draw from the GPU, which results in a lot of heat and near-constant fan noise, even while idle. As there isn’t a monitor in the world that matches the resolution of the built-in screen, this means the clocks are always maxed (unless you close the laptop and run in clamshell mode).
The 5600M uses HBM2 memory and doesn’t suffer from the issue, but a £600 upgrade to work around the issue is lame, especially when I don’t actually need the GPU, you just can’t buy a 16” without one.
Disabling turbo boost does help a little, but it doesn’t come close to solving it.
My memory is hazy on this but I did come across an explanation for this behaviour. At mismatched resolutions or fractional scaling (and mismatched resolutions are effectively fractional scaling) macOS renders the entire display to a virtual canvas first. This effectively requires the dGPU.
Your best bet is to run external displays at the MBPs resolutions and because that is not possible/ideal you are left with choices of running at 1080p/4k/5k. macOS no longer renders crisp on 1080p so 3840x2160 is the last remaining widely available and affordable choice while 5K is still very expensive.
Hardly anyone makes 5K displays - I have a pair of HP Z27q monitors made in late 2015 that are fantastic, but I had to get them used off eBay because HP and Dell both discontinued their 5K line (Dell did replace it with an 8K, but that was way out of my budget).
Part of the reason for 5K’s low popularity was its limited compatibility: they required 2xDP1.2 connections in MST mode. Apple’s single-cable 5K and 6K monitors both depend on Thunderbolt’s support for 2x DP streams to work. I’m not aware of them being PC-compatible monitors at native resolution yet.
I love 5K - but given a bandwidth boost I’d prefer 5K @ 120Hz instead of 8K @ 60Hz.
I am a bit curious to know why this specific problem has been appearing in various Nvidia lineups in the beginning of the decade, and is reappearing now.
