This depends on the way you setup and you are running the cron.
If you enabled the
Run cron every field at
Admin -> Config -> Cron (admin/config/system/cron) then the cron will be triggered by opening pages on the site, and yes it will affect the page load of visitors.
However, it is recommended to disable the cron on that page and setup a cron task on your server. You can read the article here on how to do that correctly. In this case it is not blocking the site directly, but it will still, partially, affect the visitors who are using the site at the time of execution.
This is because often the cron will look for updates and perform other resource-intensive tasks and there will simply be less server resources available.
For this reason if you need to run the cron only once-twice daily, you run it usually during the night or some other time when you don't have many visitors.
Lastly, check Elysia Cron which allows you to finetune what exactly is executed and how often - it calls selectively
hook_cron() of the modules you have on the site.