MPM Event allows Apache to run multi threaded, and thereby serving more visitors. It has to be paired with a separate PHP service, now commonly PHP-FPM. Prefork will restrict Apache’s and PHP’s performance if your server is busy. If Event is using too much, it is most likely a poorly configured FPM over Event, but an unoptimized MPM Event could be a problem, too.
Prefork works fine for servers without a lot of simultaneous traffic, but the standard for Apache is Event/FPM nowadays.