Because our gear is meant for industrial control, its got some 'snubber' circuitry on the output.
The roll of this snubber is to limit the damage a noisy relay or device can cause to the module.
The upside of this is that we can offer lifetime warranties on most of our modules.
The downside is never really an issue, till you get to bit banging serial data from a digital module..... it makes them a little slower than something like a pure digital output.
So, I think it might be best to move from the idea of bit banging using a digital module to what we need to do it right, use a converter that will allow us to use a faster protocol.
This PDF might be worth a read.
Failing that, you might need to look at using some middle hardware to convert and buffer.
In other words, given the clock frequency you mention, we need to look at using the right tool for the job.