When you place an order in the Treasury, the funds are debited from your Uninvested Cash balance immediately. In most cases, the order is then executed right away and the asset appears in your portfolio.
For some products, however, there can be a short delay:
Stocks, ETFs (including iBonds) — market orders usually execute instantly during trading hours, but in rare cases may take longer if there is low liquidity.
MMFs — orders are processed once per day, and settlement may take up to three business days depending on the fund.
Crypto — market orders are executed instantly, while target price orders only execute once your chosen price is reached.