I use Apex Trader Fundings' trade copier, they made one for NinjaTrader, and I've never had that issue where some orders don't fill, and then some fill the wrong way from the ATM, I mean, sometimes there is lag, but in the trading community I'm in, no one else has my issue apparently, or most of them just don't trade 5 accounts, idk.
This isn't an advertisement for them, I have no affiliation, but I am pretty pleased with the product they put out, just the input lag when I go to press market order 2 or more times in succession. But some dude with 20 accounts said he has no issue with that, so it may just be a me thing
I have a 5 year old MSI laptop, with 16gb of ram, intel i7, I've even over clocked my ram, it helped a tiny bit, but it's time for a new computer for me

Idk, look at reviews for a couple different copiers, or I'm sure plenty of people in this thread gave their preferred one, see which one satisfies your trading style