Most work, especially software development requires frequent communication. Being able to quickly bounce ideas off people and verify your plans before committing to them is so valuable. I can't count how many times I have been right in the middle of something, had to make a questionable choice, spoken to a coworker just to verify the idea, and they tell me something that completely changes the outcome. Being able to get this done within the hour vs getting it right at the end in review is so valuable.
This hasn't been my experience. Long periods of deep focus are more important than constant connection.
If you need to wait a day to get a response from anyone, you either have to drop the work and do something else while waiting (breaking your deep focus). Or assume you are right while waiting and risk throwing out a days work if you are wrong.
for me, unless we are doing pair programming, i expect communication to be async, even if we are in the same office. you don't get to interrupt on demand, but you better wait until i am mot focused on something else, and respond to your message
Which shows in the contributor stats where 1-3 people do all the real work and everyone else submits trivial patches.
it's worth mentioning here that GSOC may have motivated a lot of projects to improve their onboarding and make it easier for new contributors to join.
these are solvable problems. and bad onboarding is not limited to async work, nor is async work a hindrance to good onboarding.