No, sir, I have a separate computer for trading, separate internet, more powerfull than I bet yours

Chances are on my side.
There are KNOWN issues with Ninja - there are various ways to overload it. Some are simple, some are not so much. All are known and documented.
Examples:
* Put a T&S into real time mode. It normally refreshes only x times per second. Real time, it will redraw (SLOW) on every tick. Good normally, fast markets see you lag terribly. This is actually documented - just some people dont realize that when they turn T&S real time. Happened to me.
* Strategies or indicators doing a singficiant amount of maths AND set to repaint / recalculate on every tick. Normally they run pretty simple. This is also the case if only little happens in the strat / indicator - for example, I use per tick data only to move trailing stops. No big problem. But if you do a lot of maths there, then the amount of work goes up in fast markets.
* Reloading historical data under certain circumstances freezes the platform until it is loaded. Happened to me a couple of times. This is partially also the case if you open / reopen / change the timeframe on a new graph and that goes up the window with CPU usage calculating bars.
It all runs down on running more complex code than average and / or a higher number of symbols. Most issues are well documented and - at times - ackknowledged by the Ninja trader staff. Read the tremendous amounts of things getting better with Nt7 - memory consumption, speed in certain aspects.
If / as long as what you do is sensible, Ninja is actually not that bad. But especialyl for non-IT people and non-programmes it is sometimes not exactly that easy to know realize how easy it is to overload the whole system with certain easy things.
I particularly like the real time T&S redraw issue. Someone may just put their T&S into real time mode, see the warning and see all is ok. Next time the market goes bonkers, you LAG. I got a10 second lag spike once, including my strategy trailing stops and putting in orders too slow - with a nice loss. Once I turned that off, acutally, I have no lag issues to start with.
That said, the other comments are right. Stuff like background proceses, internet connection etc may use up resources. Turn of BitTorrent while you trade, or get a separate uplink

No anti virus please - WHILE you trade, you are not supposed to download anything, so you can turn off anti virus DURING TRADING. Or do as I do - use a separate computer, ONLY for trading. NOTHING else installed.