If you delete older transactions, you will need to change the amount of the Opening Balance of the register to compensate for the deletions so that the Ending Balance stays the same.
If you delete a transfer transaction in one account, Quicken will delete the other half of the transfer, which is probably not an issue since you would probably have deleted it anyway based on its date, BUT, it would be a problem if it was a transfer to an investment account which for historical purposes you don't want to delete since it might be part of an active lot.
Be careful, make a backup first.
Take a look, in Q Help, at “Yearend Copy”
WHY - it doesn't make anything faster or even that much smaller of a QDF file -