Can that be confirmed as an explicit effect of stunning versus the speed reduction? It seems like all forms of speed reduction massive reduce the threat level of foes in general. A hydra is death on a stick even without sheer attack spam and statistics (a lucky hit will occur eventually) on its side. A winded, nauseous hydra that can't stand up is nearly harmless.
Perhaps as it's not all that clear since you can't achieve a stunned status effect without the speed drop. Also in your hydra example, winded and nauseous are both negative status effects like stunned. They too cannot be divorced from the speed drop.
Anyways, let's look at speed drop by itself. Once again, pit two dwarves against one another: Side#1 NPC and Side#2 you.
On Side#1 NPC, defensive stats set to grandmaster.
On Side#2 you, shield and dodge set to grandmaster, *but* armor user left as dabbling.
Like the first test, weapon stats (sword at expert) for both sides set several levels lower than grandmaster defensive skills.
Everyone wears full metal (iron) armor as before. Weapons are wood. Two changes to Side#2 you: shield and greaves are platinum. Result is walking speed for Side#1 is 1.000, but you Side#2 is .311.
What you'll see is speed drops do not result in the same number of defensive failures as stunned. There is the difference that stunned halves speed from 1.000 to .500. Here you're not stunned, but you are at .311 speed which is much lower than the NPC's 1.000.
All significant speed drops can produce a defensive failure (in block and/or dodge) similar to being stunned, but to lesser degree.
These tests are both really sloppy excursions into !!science!!, but both roughly indicate that stunned compounds the negative for a speed drop. What the specifc negative skill drop % for stunned itself is is unknown like that of winded or nauseous.