When you add an old descriptor/key to your Bitcoin wallet it actually initiates a blockchain rescan to update its internal UTXO set and reflect the correct balance. The process may actually ttake hours because your wallet is re-processing the entire blockchain, checking for any transactions related to the newly added key, and updating its wallet database accordingly also maybe because you network connection is a little slow.

In contrast when you broadcast a Bitcoin transaction the network checks the mempool for duplicate transactions, verifying the transaction's TXID and hash, checking the blockchain headers for proof-of-work validation, and using SPV  to verify the transaction's inclusion in the blockchain. These actually enable the network to efficiently prevent double-spending.

Op also note that the speed of your confirmation depends on the depth location of your transactions in the MEMPOOL.