Regular wallets can send at most 4 transactions at once, and they need to sign the current seq_no. This in effect limits them to 4 transactions every 5 second, and this operation need to be serialized, meaning that there needs to be a single service sending these 4 transactions.

Highload wallets can send up to 254 transactions in a single request, and they don't need a seq_no. This way, they can send many transactions, multiple times without needing to wait between each batch, and there is no need for a single service, the operation can happen in parallel.

Read more here:
https://ton.org/docs/participate/wallets/contracts