Aniframeis an HTML document that is embedded inside another, such document on a website. In other words, an iframe can be used to insert a piece of content from another source into a webpage.
There are certain scenarios when using iframe within your pages is fine, such as when you have relatable, externally hosted content (an image, embedded video, short instructions, small decoration to name a few), you wish to have on your page.
However, the constant use of iframes can affect your ability to customize your pages. CSS on your site cannot affect content within an iframe, because it is hosted elsewhere. An iframe should not be used as an essential part of your site, but rather as a piece of content within it. For example, if you have an external hosted page, it is advised that one should not iframe it within another to save time and resources on recreating it.
Additionally, any time an iframe is loaded, scripts on the iframed page load. This can inflate Google Analytics or HubSpot page views of the iframed page.
Iframes are used to show information from one website on another. examples are iframes iclude:
analytics snippets
video (youtube, vimeo, dailymotion, etc.)
careers plugins
investor information
interactive third party services (Hubspot forms, etc)
etc.
basically an iframe (or embed code) is an easier version of hooking into an api to pull data. I don't know why you would want to use an iframe for a slider but if it is because the slider is being provided by another site then typically that site will provide an iframe code to paste into your site.
If all of the information in your slider is native to the page you are working on then I would suggest a simple set of divs and the use of slick.js. http://kenwheeler.github.io/slick/
All you have to do is download the files for slick.js, find the slick.js and slick.css files and plug them into hubspot in the design manager (give them their own files), then in a custom module, in the right sidebar, attach the css and js file. after that you can use any of the examples on the slick.js examples page and it should work. This is my go to slider script and I have used it for years so I know it will work for you.