Why data preview is much faster than real scraping?

How to?
#1

Describe the problem.
When I click “data preview” the result shows up immediately ,but when i click “scrape” it takes a long time to scrape the website.
I want to know why and how to speed up the process, thank you very much.

Url: http://www.itcast.cn/channel/teacher.shtml#ac

Sitemap:
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#2

Because when you are using the data preview, you have already rendered the page, so the data can be previewed much faster.

When you start a scrape, there is a 2 second Page Load Delay and 2 second Request Interval, that is there to make sure that the page loads properly, before the scraper tries to extract any information, otherwise the scraper will try to scrape the page before it has rendered, will find no information and stop without extracting any data.