is it a portable NVDA or an installed one?

BTW, have you experienced that the newest BTSync  web based interface is completely inaccessible with NVDA?

It's not the fault of NVDA but  the BTSync developers who have elected to unlabel all buttons.

Could it be caused by running too many add-ons on your NVDA? I heard somebody on this forum reporting problems when installing many  add-ons. Uninstalling and reinstalling it again probably would help, but it's too worrying installing those add-ons again.

There's a few things I've seen that can cause this.
First is add-ons.  I know this personally.  One of the old versions of Unspoken made NVDA sit and stay at about 13%, always.
Second is running next.  Next builds are experimental and also not compiled fully in release configurations.  Being on next is not likely to cause this in the long run, but can in the short term if there's a bug.
The third is more complex and much more difficult to diagnose.  NVDA is doing what is called interprocess communication.  This does not necessarily increase the CPU load, but will sometimes cause the symptoms you're describing.  What can happen is that another process has a poor implementation of the accessibility API or is continually busy, and NVDA is trying to make calls into it.  These calls then block, which in turn causes NVDA slow-downs.  A particularly bad offender for this can be iTunes.  This last is the most common cause I've seen for actual NVDA lag, but it doesn't have to mean CPU spikes.  Unspoken sat at 13% on my machine for days until someone pointed it out to me, and I never noticed because my NVDA was still running happily along with no hiccups or anything.  Open iTunes or start a long-running process with a GUI that decides to make itself realtime priority and, well, boom.
Now, a note.  Other screen readers like Jaws split the processing power among multiple processes so they can talk to 32-bit only synthesizers, namely eloquence.  When you start making the screen reader talk, it always spikes.  That number in task manager is therefore sometimes caused by the screen reader reading the stuff you did to open task manager.  Unlike, say, Jaws, the actual processing power for synthesis isn't in another process, so you see it as well; in addition, NVDA is in Python, so its CPU spikes can be a bit bigger.  But it doesn't matter anyway, and you can't really equate it to your computer in the way you think you can.  If you really want to see if NVDA CPU time is reasonable, you need to go add the total CPU time field to task manager and monitor that instead.
And finally, the developers do do stuff about it.  But you can't really put this under one ticket.  The way the issue is being described here is of absolutely zero help.  You have to figure out how to reliably trigger these spikes, and go from there.  Telling me my software is slow does nothing at all, because obviously it's fast enough on my machine.  The NVDA people can only make the improvement if you can tell them *why* it is slow on your machine.

That doesn't sound like an NVDA problem, not specifically.  That sounds like something more general is going on.  I have NVDA crashes all the time, mostly because I'm a programmer and programmers tend to do odd stuff sometimes, but it has never, ever locked me out of the machine.  I've seen that kind of symptom with jaws, however; a particularly bad offender used to be Jaws and Firefox.  And I've seen that kind of symptom when Explorer crashes.
When that kind of thing happens, getting someone sighted to look at your machine before you force restart is probably not a bad idea.  They might be able to tell you what's still working, if anything.  Also, force restarting is a horrible thing to be avoided at all costs; every time you do that, you risk breaking Windows at worst.

Hey There. Okey i was about to do homework but this happenned again and i decided it was time to seee if something can be done.

First off, i don't know if the addons i use could be a part of the problem... Going to try to remove some just to see if it happens. Second, what happens is that in some times, mostly when browsing my documents in windows explorer NVDA stops working. Not just that, nomatter which key i press. Nothing works... But i've noticed if i have something elce (licke being in a skype call, listening to music or a game which i was playing), keeps working. But i cant interact with those programs. I cant close them, or get at them so... What key works its alt, plus control, plus, delete. The only thing which fixes the problem its logging out... Clossing the sesion, and all of the programs. I get an error message and, when i am able to get sighted help to read it... Says: NVDA helper remote or something , injection terminate and some numbers.

And third, i cant seem to replicate the things to cause the problem. It could happen right now, or in a day, even could not happen at all in a long , long time. But it allways gets back... Of course in a not so good momment.

I'm running Windows 10, and it happens annoyingly often. Luckily, in 2016.2, the exit sound plays, and NVDA just crashes, and bringing it back is as simple as hitting control alt n. It doesn't lock up the entire system like it used to, at least not very often. I honestly can't believe this hasn't been reported to the devs if it's a known issue, which it appears to be.