Loading a font can cause multiple things including load time delay. For example, connecting to Google and downloading a Lato font is around 4-5 requests + js + CSS files + 500kb added download.

Processing the font itself also is at least 500ms to a second depending on the device type.

We keep the total requests under 25, which cannot be done after multiple 3rd party fonts are added.

Additionally, some fonts can introduce CLS (content layout shift) after the font is loaded. Especially H tags because they shift a lot. CLS = your SEO is dead.

For speed purposes, especially mobile, using a 3rd party font is SEO suicide.

The sites we build for our clients are below 1.2 second mobile and 0.2 second desktop load time which gives these sites an enormous SEO advantage. These load times are complete loads, not initial draws or TTFBs. (results are provided by Google page speed insights - https://pagespeed.web.dev/) These load times change when we use the client's own fonts or the ones they pick from Google fonts. We have this discussion almost every time we onboard a new client.