What is the cause of slow roll off of windowed filters
A (non-full-width-rectangular) window attenuates data at its edges, thus narrowing the width (decreasing the 1st moment) of the bulk hump of the input.  Decreasing width in the time domain increases uncertainty in the frequency domain, which is represented by a filter response which has a shallower transition band stretching farther from the center frequency.

One way to roughly compensate is to start with a wider data window (longer FIR), which when "narrowed" by a non-rectangular window function will constitute roughly the same bulk "width" as before the widening.