Yeah, the issue is that when large amounts of matter gathers into the accretion disk of a black hole it creates more and more friction as it falls in. This fric

Yeah, the issue is that when large amounts of matter gathers into the accretion disk of a black hole it creates more and more friction as it falls in. This friction generates a lot of heat (and therefore pressure) which pushes back on the in-falling matter, slowing down the accretion process. If there's an overwhelming amount of matter the accretion will disk will get so jammed up and energetic the black hole will start firing off high-energy relativistic jets from its poles. If you start with a stellar black hole of say 10 solar masses and then throw millions of solar masses of material at it, the above processes will slow things down dramatically, and you won't be able to get a supermassive black hole instantly.