Like sibling comment is saying - async inserts are part of the oss version. Batch flushes are tunable in two dimensions - size and latency. I think it’s likely the default size was just too low for your usecase
Forget the exact details, but we ran into issues with tuning the two setting you're describing on the Cloud instance (these settings were user level settings that seemed to get reset by ClickHouse Cloud because they were updated based on the cluster size). Perhaps this could change with Cloud and isn't an issue with the OSS version.