Seafile

Seafile has been in development since 2012 with one explicit design goal: make file sync fast and reliable at scale, which it achieves through block-level deduplication and delta sync — only the changed blocks of a large file transfer, not the whole file, a real advantage over naive whole-file sync when your libraries contain large CAD files, video, or design assets. It also supports genuinely client-side encrypted libraries, meaning the server never holds a usable copy of the data — a step beyond most competitors' "encryption at rest" claims.

Beyond raw sync speed, Seafile includes SeaDoc for real-time collaborative document editing, a dedicated Seafile Drive client that mounts libraries like a local disk instead of syncing everything, and per-library Markdown wikis for lightweight documentation. The written-in-C core is a deliberate performance choice, though it does mean a narrower third-party plugin ecosystem than Nextcloud's PHP-based app store.

The honest gaps: Seafile is a storage-and-sync specialist, not a collaboration suite — there's no built-in chat, calendar, or contacts sync, so teams that want those need a separate tool (or Nextcloud instead). The free Community edition also holds back a few features for the paid Professional edition, notably LDAP group sync and file-server clustering for very large deployments, though the core sync/share functionality is fully usable without a license. For teams whose bottleneck is genuinely sync performance on large files or big libraries, Seafile is the more focused, faster option; for teams that want one platform covering storage plus chat and calendar, Nextcloud's broader scope is the better fit.