Making a paper wallet "hot" by restoring your mnemonic seed in the GUI or CLI it needs scan all blocks for outputs that match your keys. You can speed up this process if you know approximately when you first created your wallet / sent the first transaction to it and check Monero's block height at that date, so your wallet doesn't have to scan the entire blockchain any more but only from that date on.
However, the scanning process is computationally intensive and it may take several hours depending on your hardware. A fast SSD with the synced blockchain and a modern CPU will speed it up. You only have to do this once when importing your seed, next time your hot wallet will know up to which block height it already scanned the blockchain and will only scan new blocks for your outputs.