I need a platform where it's possible to create demo account and (important!) with the most frequent data updates. Would be happy for any links.
Think or Swim is a very nice trading platform with lots of features including Paper Money (demo account) You have a choice to use it for 60 days without opening an account. But, If you open a TD account you do NOT have to fund it so you could use PaperMoney in Think or Swim with no restrictions on an unfunded TD Account. https://www.thinkorswim.com/t/innovation.html
Thanks a lot, I'll check it out. What about its refresh speed? And what frequency is considered normal for demo accounts?
Speed is pretty good. Also, If you open an account and agree to exchange documents you will get real time quotes. Again, you don't have to fund it.