I have seen this topic discussed too many times. I assume that is why you will get few serious answers.
Because I can't think of anything funnier than the things already mentioned, I will just recapitulate
the general opinion of previous surveys.
There are basically 3 elements to "pace".
1. The speed that the ball travels through space. The speed of the ball through the air is determined by,
several factors- but, to somplify, let's say force applied to the ball, which is then transmitted to (impact) the
opponent's racket.
2. The spin of the ball, added to the speed, also imparts energy to the ball which can also affect the oppoent's handeling of the ball.
3. The "psychological" aspect. The ability of the opponent to judge the various forces applied to the ball can make the actual
"pace" seem even greater when he/she is not prepared to control that force with their own racket.