Websites get slower over the years because things get added and never properly removed. Old fonts, duplicate analytics code, scripts from plugins you've deleted, and photos far bigger than they need to be all load on every page, even though none of it shows. You can find it in five minutes with PageSpeed Insights.
My mum's website was the example that taught me this. It was loading eleven fonts, and its mobile score was 19 out of 100.
She runs yoga classes in Surrey. The site looked fine. It took about six seconds to load on a phone, and she'd noticed fewer people finding her when they searched for yoga classes near them.
When I ran it through PageSpeed Insights, it scored 34 on desktop and 19 on mobile. Underneath, on every page load, it was fetching:
None of it was visible. It only made the site slow.
Because builders and plugins make adding things easy and removing them hard. Every widget you try, every theme you test and every "install this for better SEO" plugin can leave something behind. Over three or four years, a simple site builds up layers of it. This happens on any platform, WordPress with plugins most of all, not only on builders.
Google says its ranking systems reward pages that give a good page experience, and loading speed is part of that. It also affects people directly. Google's research found 53% of mobile visitors leave a page that takes longer than three seconds to load.
Finding the problems took an evening. Cleaning it up on the old platform helped, but it crept back every time something new was added. So I rebuilt it as a simple site with no page builder and no plugins, with the same words and photos.
The new mobile score was 99. Within about three weeks she was back on the first page for yoga classes in Surrey, and enquiries picked up again. That's one site, not a study, but it's the clearest before-and-after I've seen.
If it's a few big images or scripts, fix them, or ask whoever built the site to. If it keeps getting slow again, a simpler rebuild is usually cheaper than chasing it every year. A four-page site from me is £300, built to load in about a second on a phone.
Not sure which it is? Send it for a free website check and I'll record a short video showing what's slowing it down. More on the wider basics in SEO is not magic.

Federica helps new businesses look established, get found and get enquiries, with branding and websites, from Woking, Surrey. She studied engineering at the University of Manchester and was a Lead UX Engineer at IBM before starting Made by Federica. More about her
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If a Facebook page or a simple builder is honestly enough for you, I'll tell you.