one should expect that a possiblity at times especially around news events.
the amount of slippage will vary depending on the amount of liquidity the market maker has access to for its clients. generally speaking, the bigger the market maker the more liquidity they have access to based on bank relationships and the smaller the slippage could be.
This has nothing to do with tightness of the markets. The stop was triggered during a freefall from 1.1767 to 1.1721 resulting from economic news. The movement happened in a split second. the trigger-to-execution difference reflects the speed of the pro market.
KS96, if you're trading the news on IB IDEALPro, you MUST use stop limit orders. (And accept the risk of not getting filled on the initial spike.) Using anything else is an exceptionally bad idea, as you've experienced first-hand.