I'm going to deviate from the community a little here and say that while sliders are rarely ideal, and frequently cause performance issues if not coded well, there are other factors - business goals, senior management directives, design concepts - that may unfortunately require that a slider remain present, at least until the trend dies out completely.
That said, when dealing with WordPress themes and plugins, the quality varies wildly. It can be hard to tell how a certain developer has coded the slider or what techniques they used to show or hide images.
Here, it does seem like there is some serious, slow resizing happening in the background. I would verify this by using DevTools to load the page and record what happens. Use the Network and Performance tabs, and see what's taking so long in the waterfall. You may be surprised where the source is coming from.
The easier option is to not use the slider you have; you can disable it if it came with the theme. There are lots of different slider plugins out there for WordPress, and some even take on the CSS properties of your theme. Do your research, and go with the lightest plugin (in terms of code and features) that is also highly rated by other developers.
Failing all of that, you can - as others have said - disable the slider, and then run the speed tests again and see what the impact was. If you're in the green again, you can consider alternatives.