I have a MBP 2017 13" connected to a cinema display. Right now I'm looking for a 4K 60hz display with USBC for charging my MBP. In that case, can both monitors work? I connect to the Cinema Display with an adapter.
Add the 2nd monitor with Apple Cinema display
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Best Answer
Yes this usually does work. Connecting another monitor with PD (Power-delivery) should not change the behaviour on your other ports. Your MacBook's specs specifically mention the ability to support two 4K monitors @60Hz, so one with a lower resolution will be no problem. I myself have been using a 4K PD (USB-C) LG monitor and a 2K monitor connected through a hub on a 2017 15" MBP.
However, your graphics card needs to push many more pixels, i.e. other graphics related tasks may slow down depening on your usecase.
Just to clarify:
Of course your MacBook is limited at some point, i.e. connecting daisychained TB (Thunderbolt) devices and using an integrated USB hub in a monitor (many monitors using USB-C have this) will at some point lead to lower performance on these devices. However, you should only see decreasing speeds e.g. for external drives connected like that but the video output should generally be prioritized, i.e. not limited due to more connected other devices.