You can do well in low pace play with sloppy footwork and sloppy strokes because the margin of error is higher and you have plenty of time. You can therefore be sloppy mentally too.
Fast paced play starts with mental focus which drives intelligent footwork and controls clean strokes.
To develop that mental focus you need to see more fast balls which means you need to play up more than down or at your own level. As you see more fast balls you build an internal reference and can focus better. Your brain sees things more clearly even at a fast pace and you can then react in a controlled manner. You can also anticipate better with a larger internal reference.
In fact recent brain research supports this idea of internal reference development whether tennis, baseball, or even chess. You need to see lots of examples of something to build that internal reference.
None of this matters unless unless you DO focus and focus intensely. In really low paced noob play you often have enough time to flick an errant ball out of the way while hitting! In fast paced play it's the opposite: you need to be 100% focused on the ball snd thinking fast. Your internal reference DB needs to guide every step and stroke BEFORE they take place. The only way to develop that ability is to hit with higher level players.
I know this because I'm a fellow 3.0/low 3.5 and play with lower level players (my wife and kids), peers, and higher level players at our club.
When I go from hitting with the wife and kids to higher level players the transition is jarring due to my inexperience. This week I went from helping my wife warm up for practice on Tuesday night to hitting against two of my team's best players on Thursday night. At first on Thursday night I was a mess. It took me a good 20 minutes to get my focus and timing back to hit well.
Better players don't need to make that transition...they already have. We lower level live in a weird border region of mostly low to medium paced play with flashes of hard hitting which throws us off, especially if we don't focus and play up as much as we can.
Only on Talk Tennis can you find people who believe
that 10 feet of lead tape has no effect on a frame...