Mining speed entirely depends on the vein it's sitting on, mk 2 miner mines at 120/minute without overclock on a normal vein and a mk1 will do 120/ minute on a pure vein.
And as for making the ore go into the splitter and having an uneven split, there's an easy fix for that. Take a mk2 conveyor to a single splitter with 2 outputs, then that's 60/min going to each side, then have each of those outputs go to another splitter with only 2 outputs which then makes 30/minute. (That's literally what I did on mine) then you can connect 2 smelters back to each other wth a merger to get that to a 60/min output, repeat on the other 2 smelters, then connect those outputs to another merger with a mk.2 conveyor. And there you have 120/minute mining as well as 120/minute smelting and travelling.