11wk old went from 20 minutes on each breast to barely eating for 10 minutes total and it takes much longer for him to latch. should i worry?

Breast feeding: That is certainly a significant change. Now one factor that could play a part is that after your breast production gets stabilized, it achieves higher quality milk, which may just be filling the baby (who is also becoming a more efficient sucker), more quickly. The real way to know is to monitor the baby's weight gain. If the baby slows down the growth pattern, there may be a problem.

Not necessarily: There are many reasons why a baby who was breasfeeding well becomes more erratic in their breastfeeding behavior, such as a change in environment or an interruption in normal sleep patterns. If the infant appears satisfied afer feeding, sleeps well, voids several times a day, and is gaining weight, there should be no cause for concern. If feeding difficulties persist, speak with your pediatrician.

Depends: Breast feeding is both nutrition & nurturing. The successful BF kid can extract >90% from our breast in 10 min so the extra 10 is more nurturing than nutrition. If the kid has decided they would like to do something else & is growing well, I don't see a problem. This may be the kid who wants to wean as soon as they walk.