Loopline: I use 40 private proxies(8connections) on vps with 2gb ram, single 1.5ghz cpu on 100mbps connection - I am getting from 130 to 160 urls/s
Are proxies limiting me from getting more url/s ? (this is scraping) or is it ram/cpu ?
Another question: When I am scraping big list (over 3k keywords) sometimes it just stops pulling new urls completely and I can't figure out a reason. My private proxies are premium and every time I check them they work and pass google.
edit: I just found out that this problem occurs when I use intitle/inurl. I watched your video loopline and at first I thought proxies are banned from using those lines BUT then I decided to test them individually and they can all still use -inline -inurl ... :s
130-160 urls sec is pretty good actually. I don't have an "explanation" for it, but somewhere along the way you run into diminishing returns. What I mean by that is, somewhere between 150-300 urls/sec is where it seems to "cap" Meaning if I set connections on the private proxies to 10 or 50 (assuming I have either 50 or 250 private proxies) I still typically see 150-200 urls/sec. So you are doing well. So probably the proxies, but if you get any more I would run them in a separate instance. Scraping doesn't use all that much memory or cpu, but you can watch the performance tab of task manager while its running to get a pretty good gauge.
When it stops pulling urls completely, stop the harvest, then go to settings >> use multi threaded harvester - and uncheck this. Then harvest again. Watch the status column. Is it 302 errors?
If you are testing them individually in a browser it works different. Because Javascript is turned on, and images, and google uses cookies etc... Google can "tell" if your using a proxy more accurately, so they loosen the ban rules. However to do this in scrapebox would require that images load, java script be on etc... and all that translates to a single threaded harvester, no muli threading at all. So that doesn't make sense so scrapebox uses sockets for the scraping, which don't support javascript, but javascript is one way google can detect proxies, and without that the ban rules tighten etc... ( in so many words)
In other words, its all a game, and our job is to learn the unwritten rules and strategies that let us win.

Often times the only way to do that is to experiment, which it sounds like your doing, but go ahead and push the envelope on your experimenting.
For advanced operators, try setting connections to 10% or 4 instead of 8. Alternatively you can leave that multi threaded harvester unchecked, which will be essentially at 1 connection and you can even turn on the delay for a couple seconds if you want. But "likely" at 40 proxies and 1 connection, you probably wouldn't need a delay.
Or... bet yet, if you wanted to push the envelop and challenge your brain some, try and figure out how to make the footprint you want, without using advanced operators. Quite often it can be done, and then you don't face these issues. The cheese has moved, go find new cheese. Phenomenal book - Who moved my cheese by Spencer Johnson - if you are in business, you should read it.
Anyway I have almost always been able to find a way to get what I want without advanced operators. For example, I won't share how I do it, but I have developed a method for coercing google into giving me all of my competitors backlinks and if I want, their anchor text too. I used to use link:
http://www.domain.com Now I don't even use an advanced operator, and I get 1000 times more data then I used to. Its not fast, and it takes work, but the results are very rewarding. Think outside the box, no pun intended.