Why is there a delay in order placement during the market opening?

The delay in order placement when the market opens can be because of one of two reasons:

• Exchange capacity to handle orders: If the number of orders exceeds the maximum number of orders the exchange can handle, then the exchange lines can get choked, and the order can take up to a few seconds to go through.
• Order queue from the broker: Every broker is connected to exchanges via leased lines with a certain order per second capacity. If the order capacity is breached, they get in a queue, and the orders can take a few seconds to go through.