It's not a 1 per second limit thing AFAIK, I could pull 10 or more a second. I assume it's because Google has multiple servers they use. The limitation is in Google Sheets. I assume Google intentionally limits it to prevent too much server load or abuse. The less ImportXML functions I had, the faster they would fill the field.
I don't know if and how Google limits it, but the forum limits page load frequency. I forgot the mention it allows a short higher burst rate though.If you exceed that, you'll see this:
