It's generally thought that beginners improve most quickly by playing slower chess (e.g. 15+10 or even 30+20). If you're a 1000 and you only play bullet, you ma

It's generally thought that beginners improve most quickly by playing slower chess (e.g. 15+10 or even 30+20). If you're a 1000 and you only play bullet, you may end up cementing some of your bad habits. Whereas with slower time controls, one has time to calculate some deeper lines and really try to understand the position. And if one spends a year getting to 1400 in rapid, one will find that one's gains from slower time controls will quickly bare fruit in the lower time controls (once you learn time management). Generally speaking, players playing bullet are more interested in having fun and less interested in gaining elo (which of course is totally fine).