The site says "Right now youʼre the only person with the page open, but you can cheat and just open this URL in another tab to see what itʼs like with others." I'm using Firefox and Chrome on Ubuntu. When I try a second tab or one in Firefox and one in Chrome my computer's network connection locks up (pings stop). I have to close the tabs and then pings to google.com come back at 30 seconds and slowly come back to normal. It appears that http://playground.nostrcheck.me/relay is the problem.