http://sparrowa7io5pz6ud3ehqzosvepbxbxt2zphmkjsylp2zgxooko23pqd.onion/docs/server-performance.html
This means that block must be retrieved from Bitcoin Core using the P2P interface and parsed for transactions that have outputs matching the address.
For a deep wallet such as that used in this test, this can be a lot of data that must be reparsed for every wallet load. For this particular wallet, 3.5GB of blocks needed to be fetched from Bitcoin Core and parsed every time the wallet is loaded!