Unfourtunately I have not done extensive testing, and encountered this issue in my main game save, so currently I've progressed too far to have a reproducible case.
But at the time I had mk1 armor, a couple mk1 batteries, two legs, TWO mk2 transformers, roboport or a shield (I was constantly switching them to take out bases, and in both cases the issue happened).
The batteries were almost fully charged all the time. It seemed to happen randomly, I just start to walk slowly even though the armor menu said I have the + 60% movement bonus. The fix was to was confirmed by running left-right constantly, while the armor opened and when I start to walk slowly I just click on one of the transformers to pick it up, them immediately put it back in and I started walking fast for a few seconds.
I do not currently notice the issue while using one mk3 and four legs in a mk2 armor, but I might do some testing in a bit later just to make sure.