The Electron UI is all local.<p>I believe the sync engine translates between IMAP and a custom JSON protocol they&#x27;ve created. I&#x27;m not sure what the performance difference is, but looking at the API docs, it looks a lot nicer to work with than IMAP: <a href="https:&#x2F;&#x2F;www.nylas.com&#x2F;docs&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.nylas.com&#x2F;docs&#x2F;</a><p>It also seems to more accurately represent modern semantics for working with email. For example, it allows using OAuth for authentication, and it has separate fields for folders and labels, depending on the backing email service. (And unlike IMAP, the labels field is actually useful.) You can also download a transaction log to make sync faster, which I don&#x27;t think has an analog in IMAP.<p>Subjectively, I&#x27;ve found the performance to be rather snappy in most cases.<p>(You can also run a local client of the sync engine, if you don&#x27;t want to trust their hosted copy: <a href="https:&#x2F;&#x2F;github.com&#x2F;nylas&#x2F;sync-engine" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;nylas&#x2F;sync-engine</a>)