Does binlog_group_commit_sync_delay affect existing binary logging in MySQL
Existing logs are not affected. MySQL won't read and modify existing logs, it would be incredibly expensive and risky. So, yes, you'll see the benefit of this in 2 days (assuming the lag won't increase or decrease).
To me, it sounds like you are trying random optimisations to reduce the lag. If I'm correct, I recommend to investigate the reasons for slowness before making changes. Once you have an idea to make the slave faster, change a single variable, see if your hypothesis was correct, and if not undo the change.
If solving the problem is urgent, there are some things you can do to make a replica faster, like setting sync_bilog=0 and/or innodb_flush_log_at_trx_commit=0. In most cases you'd see the replica start to catch up. However, note that this is risky: if your replica crashes you will lose data. Depending on how unlucky you are, you may have to import a new backup from the source.