Reduce Bootstrap Execution Time

When syncing the new Spaces Subnet (https://testnet.avascan.info/blockchains?subnet=Ai42MkKqk8yjXFCpoHXw7rdTWSHiKEMqh5h8gbxwjgkCUfkrk), it takes over 8 minutes to execute the first 30k blocks.

It is likely most time is spent in the unoptimized `lookback` function. There is a lot that could be done there to improve performance.