My understanding is that the 84/85 were slugs for the same basic reasons as the 90~93 models were: all of the power producing parts were designed to work at different points of the rev range. Not that the fundamentals were wrong, or that any single part is "wrong", just that the parts didn't work together.
I've had a couple of YZ125Ls. I didn't think they were that bad, but nor were they bikes that I found easy to love.