For a few weeks, I've been using a very basic option that had helped me to investigate this issue : the setting that displays the number of FPS at left. Most of you may use it already but I think it's interesting to share my experience about that.
I indeed discovered several things : the rare times I experienced these slow conditions it was when my "Frame Per Second" were blocked at 58,8 fps instead of 60.
Under these conditions, the whole game changes and lacks of fluidity : the ball for instance becomes far less fluid. This has a huge impact on your positioning and also on your timing to counter and hit correctly the ball. It also decrease your ability to anticipate and defend well because your eyes are disturbed.
What is very weird is that this happens to me even when I have a lot of free memory available on my computer. It seems it happens when I don't close TE for a time, or when I join another server after having played a set.
The best solution is either to relaunch TE or to restart : as soon as you reach 60 fps at max, the game becomes perfect again.
I don't know if this 58,8 fps stuff is a glitch with the use of the memory of the game, but beware of this. If you feel the game doesn't run smoothly, just restart, kill some Windows processes/applications or other stuff that turn in background like Mozilla as it may have an impact.