An LCD monitor uses a scaler to convert everything it's fed with to the panel's native resolution. This step takes time. Additionaly, if you fed it with an analog signal (VGA), it will need to do analog to digital conversion. That's more than 7ms.
The optimal situation of course is to always use the LCD's native resolution through a digital output. I believe that modern G-Sync/Free-Sync monitors should be able to start lighting the pixels as soon as the information is received without any buffering /processing.