There've been some interesting posts here. It's pretty easy to get into a kind of swaying rhythm with the form, where I could count cadence in 4/4 time. The "end point" of each forward or back motion is a beat...
HOWEVER, I don't think this is what's meant by even speed. I suspect the ultimate goal is to have the form flow so seamlessly that there is no rhythm. After all, isn't rhythm another kind of form that can eventually be dissolved?
I try to stop myself when I fall into a rhythm (unconsciously) because (for me) it means that I'm not paying enough attention. When I have a rhythm going, it often means that I'm doing the form like a wave--that is, part of me is arriving at the shore (of each end point or transition point) before the rest of me. It's like I'm sloshing around. There's too much of a to-and-fro flavor, even if the outward shape of the postures hasn't changed.
Since one of my goals is to narrow the gap between when the wave originates and when it arrives, then the rhythm has to either go away or smooth out into a very fine vibration that's contained throughout the movement. (I'm not really sure which.)
It's the difference between sloshing a glass of water around and trying to wave around a glass of ice.
When I try to copy the feeling of what I perceive YJ is doing, the internal sensation of rhythm goes away (even though there are still distinct transition points and end points in every movement). The feeling of speed remains constant (on a good day), and even the tiny internal transitions feel like they are happening at the same speed as the larger external movements.
Moreover, it feels closer to being all one unit. Everything can unify and I arrive all at the same time. Even though I'm moving through space and time, I'm all together with myself throughout each movement. There's nothing lagging behind. When I can do this in push hands, I can change much more quickly and acurately.