There some assumtion to it
This is based on my experience & assumtion, so no scientific fact or the correct answer.
I own 2 racket i mainly use. Victor TK770 3U & Adidas Wucht P8 4U. I smash harder with TK770 yet faster with P8.
So here the assumtion:
1. Shuttle hold
When i hit a shuttle, TK770 feel had slight delay before the shuttle bounce back, while otherhand P8 almost instantly goes away. The hold could cause the slowness but as its like charging all its force, when bounce back it carry full energy from you kinetic chain movement.
2. Sound
TK770 smash sound KABOM!!! Yet P8 abit dampened. Coz badminton are fast games that rely alot on reflex, with dampened P8, opponent dont know when to react thus feel like its faster. While TK770 with its shocking sound tho its more powerfull but it give clear input to opponent to react to it, so its feel slower.
3. Shuttle spin
Spinning shuttle can cut through air easier thus makes it faster. While non spin one forcing itself breaking the air barrier so its slower but as its forced, it carry alot of force thus feel heavier.
No one know the right answer as there is some thread discuss it yet no solid proof for every argument & im to lazy to experiment with it but just enjoy the sports.
Hope that could lighten your thirsty for knowledge alittle bit.