Why is the resolution lowered when multiple monitors are connected simultaneously?

The DisplayPort Multi Stream Transport has the ability to allow multiple external monitors to be connected to host devices such as laptop via a USB type-C connector and display different graphical content. There is, however, a limitation in how many pixels can be sent down the display pipe.

When adding a second external monitor via a type-C MST hub, both monitors must drop the resolution down to share the bandwidth.

That said, it all depends on the PC’s native GPU performance and the DP version it supports.

① DisplayPort 1.2 should allow connection of two 1080P monitors.

② DisplayPort 1.4 should allow connection of two 2K monitors.

③ DisplayPort 1.4, which supports DSC 1.2, should allow you to connect two 4K monitors, one running at 60 Hz and the other at 30 Hz.

Display Stream Compression (DSC) 1.2 supports visually lossless compression of ultra-high definition (UHD) displays while also supporting high dynamic range (HDR).

In general, if the external monitor is not up to the required resolution, such as 4K@60Hz, it depends on how much bandwidth the computer's DisplayPort version supports as well as the performance of the native GPU output.