The difficult thing to reproduce about it is, it happened several times under varying situations and I didn't notice until it had accumulated a few times. When I finally caught it, it appeared to be about 11% reduction each time. The character died six times, though, so at 11%/per, one of them didn't do it; he was down to 45% global speed when it finally became too much to manage. What I do know is, the slow was visible as soon as I was on the Eidolon Plane, but there was no debuff listed. Since I was trying to not-die, I hadn't thought to check my character sheet immediately before death.
I'm still not entirely sure what exactly is doing it, but a quick test with this addon disabled didn't have that problem. But, see above; one of those deaths didn't either, so it may still be unrelated.
I'll have to mess around with it more later. The character was an Orc Archer in Embers of Rage. I do tend to use a lot of addons, as you can see
, but these deaths were in a pretty wide variety of situations.