Do custom web fonts slow my site down?

They can, and the visible symptom is text that flashes or shifts as the page loads:

• Each weight is a separate file - loading four weights and two styles is eight downloads for text.
• Third-party font hosting adds a connection - self-hosting removes that round trip.
• font-display controls the flash - swap shows fallback text immediately rather than leaving a gap.
• Layout shift is the real cost - text reflowing when the font arrives affects Cumulative Layout Shift.

Two weights of one family covers almost every small business site. Everything beyond that is paid for on every visit.