Do WordPress cron jobs slow down page loading?

Short answer – Nope. Any page request initializes the scheduled queue. It’s just an initialize request. Wp-cron request is a standalone request.

so requesting URL /somepage you just initialize request to /wp-cron.php

However – If cron event doesn’t work really well (it’s has 1000 db queries e.g. or its requesting a some really long-to-respond resource), or both, or re-scheduling cron event for each request… just like any other http request it will eat resources, CPU performance, memory, etc… if it eats enough resources, your page will become slower.