I'm no transformer expert, but i'm nearly certain the "primary capacitance" is the main cause of top-end roll-off.
Do you mean the
parasitic capacitance of the windings of the transformer primary? As
@CJ very pertinently pointed out, capacitances are usually specified
between two elements.
https://sound-au.com/articles/audio-xfmrs.htm#s1
"A transformer's high frequency response is limited by leakage inductance. This is caused by magnetic flux that manages to 'escape' from the core, and it appears as a separate inductance in series with the primary. There are ways that are used to minimise leakage inductance, and these must be applied in any transformer that has a significant step-up or step-down ratio. 1:2 or 2:1 ratios (or less) are usually easy enough to make with acceptable leakage inductance. While measuring leakage inductance might seem to be a rather esoteric test, in reality it's actually quite simple - short circuit the secondary and measure the primary inductance."