The ferrite will lead to further 'rounding' and in either case I would put a schmitt trigger at the lcd to 'sharpen' the signals. This will also improve noise rejection.

The ferrite is not meant to cure any timing degradation, of course. Means, in any case the timing must be within acceptable parameters even without ferrite.

A big advantage of common mode filtering is that signal bandwidth is much less degraded as compared to standard line-to-ground filtering (differential filtering). But nevertheless, there's also a bit signal bandwidth filtering due to imperfections of symmetry of common mode choke. So, there can indeed be observerd some slight rounding of edges.

I think, the most rounding will come from 'dispersion' of cable or mismatching of characteristic impedance. So, to use this Schmitt trigger is a good recommendation. By the way: 74LS244 and 74LS245 always HAD Schmitt trigger inputs...

Kai