In taraweeh, the imam recites so fast that by the time i finish sana-audhubillah-bismillah, he finishes surah fatiha. Is it allowed to read only bismillah or should i complete the sequence no matter what?
All perfect praise be to Allah
Firstly i will suggest to talk with imam or switch masjid, and pray taraweeh behind some reasonable person who recite quran not fight with it...
Secondly, sanaa/istiftah is sunnah, if it is left because of some reason, than prayer is valid. Like in a situation you join salah and imam is reciting surah al-ikhlas and you know if you skip istiftah you will complete your fatiah and hence your rakaah will be valid, so you will skip istiftah in that case and you salaah is valid.
But if you know you will recite the fatiah after istiftah before imam goes to rukkuh, than you should try to recite fatiah and leave istiftah.
Reason being istiftah is sunnah and fatiah is wajib.
Have a look at here