According to codex,
Schedules a hook which will be executed by the WordPress actions core on a specific interval, specified by you. The action will trigger when someone visits your WordPress site, if the scheduled time has passed.
Assume that I run a function using
wp_schedule_event. Function need 20-30 seconds to finish their task.
So it will affect to the user or not? I mean if it is run in background like normal cron jobs it is not affect to the user.