What's actually happening
When you place a market order, your platform sends it to the broker's server. The time that order takes to travel - your round-trip latency - is time during which the market is still moving. If NFP just printed and EURUSD is moving 30 pips in 2 seconds, 200 ms of round-trip time corresponds to 3 pips of potential slippage from network alone.
Add to that the broker's execution processing time and any queue depth at their server during peak load, and your total slippage exposure grows.
This is why brokers who advertise "no slippage" accounts are technically accurate in some conditions: if the broker always fills at your requested price regardless of where the market moved, slippage is zero but the spread is wider to compensate. Different execution models trade one form of cost for another.