Why is my two-stroke hedge trimmer not running at full throttle and why is it quieter than it used to be?

This is typically an indication of a restricted spark arrestor. This causes lower RPMs and quieter than normal running conditions.

Spark arrestor maintenance

What is the purpose of a spark arrestor on my hedge trimmer?

Husqvarna recommends that the spark arrestor is cleaned after every 50 hours of use.