NinjaTrader 8 is a big upgrade to NinjaTrader 7. NinjaTrader 8 is multithreaded and was designed to take full advantage of modern PC architecture. To achieve the highest possible level of performance, it utilizes all available CPU cores, additional memory resources, and recommended to only run on SSD.
I too have been frustrated in the past, so I've spent 3 years building my own software. I gave up on Python, its way too slow for my personal requirements. Instead I've written my platform from scratch in Golang. I utilise a web based UI, I use Bulma as my CSS component library and Highcharts for visualisation. I use a Golang based object database (Pudge) for persistence. I use IQFeed and IB as a datasource and IB and IG Index for execution. I've built my own custom data collection, backtesting, risk management, execution, monitoring from scratch. I download and analyse 1000's of assets, which I then form into portfolios for trading. No open source, or even paid application comes anywhere close to my requirements. I believe that if you're serious about quantitive trading you need to build your own software. Trading via a generic platform and via charts is not for me.