Yes it is very hard to get speed gear with speed substat. 

It is actually very hard to get good gear,period.

I have 1 mill player power and face off teams in silver 3 with a non speed tuned unconventional go second team. I still win the majority oof the obligatory 5 arena fights.

If you're running dragon 20 it's not possible to get 3 and 4 star gear.

If you spend even a week just doing your auto battles in dragon you will get a full speed set, maybe not the best but a good starting point to move up from.

Hello OP

this is not an answer to your question directly, however it is a demonstration that with 10 runs in each stage on chapter 6 in brutal difficulty you can get some results like the image describes

doing the same in nightmare will produce the same (and better) effect

doing this in the boss stage of the campaign will also produce even better effects as it drops higher rarities

doing this in dragon, again, is much better, however, with dragon we need to take into account the amount of sets there are and how many possible drops there will be because of that.

I will not deconstruct your post as I do realise there are some missing pieces and it is indeed frustrating. Thus I will leave you with these words and this image

i

if you have any question otherwise, feel free to ask away

you got this!