Brachos 21b

The gemara says that if a person goes to shul and finds them davening, the rule is that if he can start and finish before the chazan gets to kedushah, he should do so.

What about a person who davens a long shmone esrei or in a shul where they daven very quickly and even if he starts at the same time as everyone else he will not be finished by the time they chazan reaches kedusha - should he start before everyone else so that he can say kedusha? Normally one is not allowed to start before everyone else but maybe here it is different because he is doing it in order to say kedusha.

See Piskei Tshuvos Page 822 for a discussion and sources.