Indexing Rate

Why do some sites have higher indexing rate and some have a very low. I have one site which articles gets indexed within few hours and one site articles aren't indexed till one week.

Well, indexing of any website which is not in our control but we can do few things which can increase the chances to get indexed faster. Like, create backlinks for the specific URL which you want to index, it is my own experience that you can get your URL index faster.

Or if your site has low authority, that may put your site lower in priority in terms of getting your articles indexed by the search engine. If you build links to high authority (quality) sites, that should increase your site's authority, and for a site that is deemed as authority, you may even get your articles indexed within an hour (or sooner) or within 24 hours. Otherwise, you can perhaps encourage the search engine bots from indexing your article by using an instant indexing service, etc.

It's all about content of the site. The more complicated the site is, the harder it will be to index it

Its according to the quality of content and Perfect onpage SEO

Content is the king always. So make your content unique and make readable one. It will get SERP easier.