For a transaction to be successful, it has to have a sequence number one greater than the current sequence number of the account funding the transaction.
Therefore, if you want to submit 10 transactions, they need distinct sequence numbers, and each one can only succeed if their sequence number is one greater than the current sequence number of the account at the time they are evaluated. This is why waiting before submitting improves your success rate - there is a higher chance of the previous transaction completing (the ledger being updated, and the account sequence number incremented) before you submit your next one.
To submit faster than this, I think your options are:
- Submit them from different accounts. You can submit 10 transactions simultaneously if you have 10 accounts.
- Submit multiple operations in a single transaction. If you can batch up multiple payment operations in a single transaction, then they can all be submitted together (with the same single sequence number).
It is true that if several transactions with incrementing sequence numbers make it into the same transaction set (ledger), then they will be ordered correctly, and can all succeed. However you have no guarantee that this will in fact happen.
Note also that the delay method is intrinsically vulnerable to racing: the delay only works because it is of the order of the average ledger close time. To really be sure, you should submit transactions with timebounds, and only submit (or resubmit) another transaction when either i) the timebound has expired, or ii) you receive confirmation of transaction success.