That 'lazy arm' feeling, I believe, results from using muscle stretch forces to axially rotate the arm using mostly internal shoulder rotation
(ISR). The largest muscle that drives ISR is the lat (latissimus dorsi). In my opinion, the lazy arm feeling occurs because you don't need to send nerve signals or feel effort to shorten a stretched muscle - unlike when you deliberately send signals to shorten a muscle without stretch. It's like using the stretched muscles - the spring - in your legs when you run.
To stretch the ISR muscles external shoulder rotation
is used. The leg thrust helps stretch the lat because when the legs thrust the racket & forearm are about at a 90° angle to the upper arm - when the legs thrust up, it externally rotates the shoulder joint thereby stretching the ISR muscles.
This Raonic video shows the external shoulder rotation that occurs when the legs thrust up with the elbow bent so that the racket & forearm are at about 90° to the upper arm. Leg thrust is not shown in the frame but it causes the head to rise up so that you know when it occurs. In addition, raising the hitting shoulder and dropping the other also stretches the lat & pec.
Several other videos in the collection show the ISR leading up to impact.
If you try to piece the serve together and practice for too long without using stretch forces you might pick up some bad muscle memory.
The serve is too fast to see or record with ordinary video. This video shows all that goes on in the 0.02 second before the ball impact.
The video shows the motion from the semi-straight arm up, the racket about at 90° to the forearm with the racket head 'edge-on' to the ball. ( To clarify, this 90° angle is at the wrist, the one above was at the elbow.)
The only way to be sure what you are doing when you hit the ball is by using high speed video. Without HS video, a list of check points for the service motion that you want is useful.
You can search the terms & related illustrations for - internal shoulder rotation, external shoulder rotation, stretch shortening cycle,