Have you considered downloading just the envelope of each email very quickly at the start (takes 2-3 seconds for me with 3,000 emails) and then lazy loading the

Have you considered downloading just the envelope of each email very quickly at the start (takes 2-3 seconds for me with 3,000 emails) and then lazy loading the rest either when they're loaded by the user or slowly over time? Even downloading hundreds of thousands of items would take only a minute of high CPU usage.