Most players split their time between formats by habit rather than choice. Your own results can tell you whether that split is helping.
Three numbers sit on your profile and look comparable: bullet, blitz and rapid. They are not the same scale, they are not measuring the same skill, and the gap between them says more about how you play than any one of them alone.
Ratings only mean something relative to the players in that pool. Bullet attracts a different crowd from rapid, and the same person can sit two hundred points apart across formats without any of the numbers being wrong. Comparing your bullet rating with a friend's rapid rating tells you nothing at all.
The gap between your own formats is informative, though. A large bullet rating and a modest rapid rating usually means fast pattern recognition and shaky calculation. The reverse means you calculate well but cannot do it quickly, which is a very different problem to work on.
None of this makes fast chess bad. It makes fast chess bad at one specific job, which is learning things you do not already know.
Compare your own accuracy across formats and you will see it fall as the clock shortens. What is more interesting is the size of the fall. A small gap between your rapid and blitz accuracy suggests your play is based on recognition that survives time pressure. A large gap suggests you rely on calculation you do not have time for, and blitz results are costing you more than they should.
Look at the gap against similar opposition, and remember that your accuracy edge over your opponents matters more than the raw number. Everyone plays worse in bullet; the question is whether you play worse than the people you are facing.
The simplest way to decide whether a format is costing you is to add up the rating you gained or lost in it over a long stretch. Not the win rate, the actual points. A format where you have lost steadily across hundreds of games is either mismatched to your strengths or being played in the wrong conditions, for example late at night or after losses in another format.
Do this per format rather than in aggregate. It is common to find one format quietly funding a habit in another.
A reasonable default for an improving club player is most of the time in rapid, some in blitz, and bullet as entertainment rather than practice. Adjust for what you are working on:
Because bullet games are quick, they dominate most players' game counts, and the emotional experience of a bad bullet session colours how you think you are playing overall. If you have played three thousand bullet games and four hundred rapid games, your bullet results will drive your mood while your rapid results are the better description of your chess.
When you assess yourself, weight formats by what they measure rather than by how many games you happen to have played.
Statmate keeps every format separate: your rating curve per format on one graph, win rate and accuracy per time control, and your accuracy edge over your opponents in each. The Insights tab shows performance by time control side by side, so the size of the gap between your rapid and bullet play is a number rather than an impression.
The rating leak detector measures the points each format actually won or lost you across your history, which is usually the fastest way to see where your playing time is going.
Treat the three ratings as three separate measurements of different skills. Use the gaps between them to work out whether speed or calculation is your limit, count the rating each format really costs you, and spend most of your serious playing time in the format that lets you think.