Is this your actual goal, or the means to some end? Do you need the ergonomics of split keyboard to deal with wrist pain, or do you just like the particular 42 key keyboard that you have?
There's not much special about typing on a split keyboard, I suspect your typing performance issues are mostly due to its miniature size and an unfamiliar keyboard layout.
So my end goal is optimization and workflow improvement. The problem is that those things are subjective. People type just fine on membrane keyboards, I can't touch them anymore.
In my case, before the split keyboard I had severe pain in my wrists, and after the switch, it’s gone. There is an adjustment period, but the benefits are definitely worth the annoyance.
In my experience: while learning Dvorak, I found it difficult to type with QWERTY.
However, now that I'm used to split and ortholinear keyboards, I can type at the same speeds with either.