Why is the trace on my DSO displaying faintly (too light) at longer sweep rates?
Check memory depth. Make sure you aren't using a low memory depth. At slower sweep rates, the number of samples needed to show high resolution is greater.

DSOs attempt to mimic analog appearance by varying the intensity of the drawn waveform based on the signal dwell time in a particular location. The longer the signal is present at a particular level, the brighter it appears on a phosphor-coated CRT display that legacy oscilloscopes used. Consider a square wave with a fast edge: The top and bottom of the wave will appear bright while the vertical transitions can be very faint.

The sample memory is utilized to interpret intensity. The more points clustered in a given area on the screen, the brighter the trace will be drawn. With long sweep times, the points stored using a given sample depth will be "spread out" more. The spread distance of those points are used to determine how intense to draw the trace. The oscilloscope's particular algorithm for comparing sample distance to screen pixel distance will determine the trace intensity.

Changing the sample depth to "Auto" or to its maximum of 14M points fixes the trace intensity for 200ms/div sweep rates and longer:



The vertical clock pulses are still faint because they are of very short duration with respect to the overall sweep time.