Ultimately, the jury is still out whether strength training, plyometrics etc. can ultimately produce much of a difference in your peak speed/power potential in regards to a light implement such as a racquet. There is contradictory data/results/theories.
HOWEVER, by training your nervous system to recruit fibers more efficiently you can develop a faster and faster swing. Which is why unmuscular pros can swing very very fast. Beside the fact they get more ball speed with their consistent timing at contact...
I think overspeed and underspeed training can be helpful. You can swing a weighted racquet but sometimes the extra weight creates dangerous force on the body. Try racquet with cover onto increase resistance without too much weight gain...or slip a shirt over the racquet head and tie it on....or even swing a knotted towl. Use your real stroke movement, you need to mimic the actual movemnt. You can also then vary this with swinging a lighter implement, for example a badminton racquet.