I the days of 'original' TTL bipolar logic, unit loadings were the main issue. With CMOS devices, however, the input impedance is quite high (10's of megaohms) so resistive loading is not the issue - capacitive loading is. So, the whole issue of loading becomes a speed/frequency and capacitance issue that is not quite as easy as the old unit loading. As one would rarely use a bipolar TTL device these days it is probably incorrect to talk of 'TTL' as the only similarity is the 'TTL' voltage levels on the 74HCT family of devices. We're living in a world of CMOS!