Invariably, when a player is caught out of position and so can't get to a ball, they think they are slow. I know players who voice this regularly, constantly yelling, "I'm too slow!" Sometimes I correct them, but usually I just shake my head slowly. Almost always the problem wasn't that they were too slow, but that their previous shot left them off balance or out of position. What does this mean?
Suppose a player steps around to play a forehand from the backhand. Done properly, the player should literally follow through back into position. If you don't, you are doing it wrong, and that's why you are "too slow!" to cover the wide forehand.
Other times a player is rushed in stepping around the backhand to play this forehand, and so ends up following through to the left (for a right-handed player), and so can't recover in time to cover the wide forehand. Again, they yell "I'm too slow!" But even here, the problem isn't being too slow; it's either they chose the wrong time to step around, or they didn't recover properly. Even when not rushed, many players finish off-balance, and so can't recover quickly. Instead, you should follow through onto your left foot (again, for righties), and use the weight on that foot to immediately and smoothly push yourself back into position..
It's not just stepping around the backhand to play forehands. You also have to play forehand or backhand when the opponent plays shots to the middle, and that puts you out of position. If you don't follow through back into position, or if you finish off-balance, you'll be vulnerable to an aggressive shot to the corners.
Placement also counts. If you step around your backhand to play a forehand, if you go down the line you give your opponent an angle into your wide forehand that you likely cannot cover. So you should only go down the line if going for a winner or with a slow loop that allows you more time to recover. Instead, in that situation, mostly go crosscourt wide to the backhand, or to the opponent's middle.
Occasionally, of course, an opponent will simply make a great shot that catches you out of position. If that happens, accept that he made a great shot and focus on making sure it takes a great shot to catch you out of position.
If you truly have a physical handicap that honestly makes you too slow (including extreme age!), then that's a separate issue - but then you should focus on staying in position, except perhaps when stepping around to end the point with a forehand. But even here, you should focus on positioning and balance so that you don't end up yelling "I'm too slow!" when in fact you should be yelling "I'm so out of position or off balance!" Doesn't have the same ring, does it? So next time, why not just yell, "Positioning!"