To find your transactions, Biscuit downloads each new block from a node and checks it on your computer. Until it has caught up, recent payments don’t show and your balance may be behind. The status bar says Synchronized once it’s done.
A wallet opened every day catches up in seconds. One left closed for months needs to scan every block since: a few minutes, longer over Tor, where the data travels through three relays. Bitcoin and Litecoin sync separately and much faster: Biscuit asks an Electrum server about your addresses instead of scanning blocks.
Help › Debug info shows the wallet height (the last block scanned), the node’s height, and the restore height.
The restore height is the block a wallet starts scanning from: nothing before it can belong to the wallet. A new wallet starts at today’s block, so its first sync is instant.