What exactly is bootstrap?

This site will answer your questions I think http://getbootstrap.com/

Using your own CSS would result in a slightly faster application, assuming you’re building a web app. This is ony because a browser ‘interprets’ source code, it parses it line by line…so everytime you open a webpage it’s actually downloading the source code to your browser and interpreting it before it shows you the site on your screen. Bootstrap is lines and lines and lines of predefined code with classes…so it’d take a little longer to download but the amount of time is hardly noticeable.

On the other hand, coding with Bootstrap would result in faster development time of your web app. In other words, instead of having to come up with custom CSS for all the various screens and devices that will be viewing your app, Bootstrap would take care of that with just a few lines of code…meaning you could get your app out there faster with fewer errors or bugs.

Thanks for the explanation guys. It’s been helpful

Bootstrap doesn’t necessarily download each time someone loads your page. The CDN bootstrap file is saved to the cache of the browser. So if you hit the cache lottery and the user has already visited a bootstrap enabled site, the browser will use that instead of downloading it again.

I’m kinda late to the discussion, but speaking of Bootstrap, here is a really useful set of libraries that help developers accomplish many various tasks easier and faster: https://github.com/Cleveroad/Bootstrap
Might be helpful.