If I say it in simple words, then I can tell it better by looking at the backend in the whole scenario. To increase the speed of the website, you first optimize the images, then if your site is in coding then go to the hosting and optimize the files. If it is in WordPress, then your files will be optimized through the plugins. You will have to look at other things, such that if someone is a professional, then he will understand these points. Like how many plugins are there in a website, you can check its WordPress backend and remove the unnecessary plugins. There are other similar things also for that I'll advice to hire a professional.