Storage Backend - Routing / Speeds

When using a storage backend, e.g. S3, should we expect clients to experience connection speeds relative to that of S3 or relative to that of the seafile server? In other words, when a client interacts with stored data, is it routed through the server or connected directly to S3? When a client downloads a new library, will the download speed be the upload speed of S3 or of the seafile server? Does storage backend encryption affect this?

TIA

The data is first sent to Seafile server, then sent to S3.