During liquid, non-news sessions, there should be very little slippage, unless market is particularly volatile.
You're probably trading through a retail shop, right?
For instance, on Interbank platforms - there's usually 200 - 500 Million per side, during major session on EURO.
1 lot won't even touch it.
Also try ping time (to broker), internet connection and RAM+CPU (system speed).
Also, if you're scalping those fast drops, yes, expect some slippage.
Forex often stair-steps and water-falls. consolidate consolidate DROP consolidate consolidate DROP....etc