The signals are sent with the help of Open High low close (OHLC) data. As soon as the candle closes, the OHLC data is used for calculating the indicators after which your condition is matched, once your condition is met, a signal is sent to your system. This is to make sure that no false alerts are being sent.
Our team has recently optimized the speed at which alerts can be sent. The latency, in some cases, is reduced to approximately 15 seconds. The team has confirmed that it might not be possible to maintain the accuracy and keep the alert delivery time at 1-2 seconds due to various reasons.
One of the key reasons being, the data latency in candle formation and retrieval from the exchange data sources itself. If we hurry and generate alerts without confirming multiple times that the candle is completely formed and all ticks have been accounted for the candle, the signal might be incorrect and later not match with the charts or backtests, leading to more confusion and unreliable analysis. You can read more about this here