None of USDT, TON, TRX, or ETH is reliably the fastest way to get Telegram Stars at every moment — confirmation speed depends on current network load, and a network that's usually quick can slow down during a busy stretch. Checking conditions at checkout beats assuming a fixed order.
Speed comparisons between a card and crypto miss the point a little — a card either declines or clears near-instantly, while USDT, TON, TRX, and ETH all clear for certain, just on a timeline set by the network rather than a bank.
TON and TRX often confirm quickly under normal load, and TRC-20 USDT usually isn't far off; ETH's speed depends most on which layer the transaction uses.
Under normal conditions the difference is often small enough not to matter for a Stars purchase — it becomes more noticeable only when one network is unusually congested.
Usually not — converting between assets takes its own time and network confirmation, which can end up slower than just waiting on the original payment.