One ledger takes close to ~5.5s on average. As of September 2021 there is around 200 transactions processed per ledger, which gives around 35 txs per second. That is current rate and nodes settings can be changed to process more if required. Time will tell how it scales up.
Due to ledger time your 6 seconds from call to response is as expected.
USD is external to Stellar, so enquiring Stellar will not work. But from the Stellar network you can get XLM/USDC rate, which you can retrieve / calculate from the order book (assuming USDC == USD)
or using https://ticker.stellar.org/ (https://www.stellar.org/developers-blog/a-new-ticker-for-the-stellar-community?locale=en)
I don't know how reliable is the ...
Timeout happens when Horizon did not receive an update for a given transaction submission from Stellar-Core. This can happen because of two reasons:
Transaction was not included in the ledger for 30 seconds. This usually means that the fee was too low.
Less likely: Stellar-Core lost sync with the network after transaction was submitted. In such case Stellar-...