Being that your system shipped with a 90 watt adapter, it will require 90 watts to charge correctly. The reason the system can be configured/ordered with a lower power adapter has to do with what processor/gpu config was ordered with the system.  For example, a 65 watt system would be able to be charged at the correct speed by the monitor, but it would also be lower spec'd laptop.  
Hi, I'm trying to charge a Latitude 5530 using a USB-C cable (certified for 100W), by connecting it to a Dell S2722QC display. When I attempt to do so I get a warning from Windows 11 stating that it's on slow charge mode.  Looking at the original power adapter it uses 90W and the monitor specs claim that it can output up to 65W via the USB- C port, which made me think that the display output may not be enough, but Dell seem to sell a 65W power adapter for this laptop. Anyone knows whether it may be possible to charge the laptop via this configuration? Or alternatively, whether using slow charge may cause any side effects, other than taking longer to charge? Cheers.
Latitude 5530, S2722QC, slow charge via USB-C?