This question already has an answer here:
How could primordial galaxies or quasars at great redshifts like quasar ULAS J1120+0641 (detected as per when the universe was 700 mill years old) have blackholes at their centers with the mass of 2 billion suns considering that early star formation in the early universe post big bang started forming after 100 to 250 million years. This will imply that this quasar acreated the mass of two billion stars in the time span of 400 million years.
- Are such accretion rates possible given distances and time scales?
Assuming that primordial stars where supermassive as suggested and black hole formation followed very fast. I guess a mass of 5 million stars should flow through the black hole every year. I have read that these proto-stars would have been 100,000 to one million times more massive than the sun. So I guess many blackhole mergers would have had to happen in that space of time to explain such large increase of mass.
- If that would have occurred would not there be signs of this events in the early universe that may be detectable as black holes mergers seem to be very powerful energy sources.