As someone who typically doesn't enjoy long music times, I get why people get frustrated by this, but I've had many friends who are worship leaders and musicians and there is a spiritual warfare element to this time.

It's to bring the whole congregation into a place to receive. You or I may be ready after 5 minutes if we're walking closely with the Lord, but highly distracted, frustrated or hard-hearted people in the congregation may need more time to soften and become "present" and open.

It's also a primary aspect of our relationship with God to give back thanks and praise and worship to Him for what He has done. It's as much of a "tithe" as finances or acts of service.