Although, themes also add some features and functionality to a site but they are different from plugins. A plugin can't change a websites design, look and feel but a theme can do everything. You can customize some functionality in themes instead of installing plugin but themes can't be replaced by plugin.

A theme is external effect like a website's presentation and designing. Whereas, a plugin is the interior effect which add or remove some functions to a site.

Theme organizes and displays the entire site's information to display it to the visitors.

If you want to change the appearance of your site by changing your theme, then all the customizations done will be gone (data and information will remain safe). But you can activate or deactivate a plugin whenever you want. It will not affect the site in any way.

Your site speed is mainly affected by designing of theme, number of images, cache power. Number of plugins installed and functionality code exist as plugin or theme does not contribute in site's speed.

It's better to allow themes to handle the look of a site and plugin to handle how a site should behave. Thus, when theme will be changed, all the functionalities of site will remain there.