Fixed vs variable spreads
Fixed spreads remain constant regardless of market conditions. Typically slightly wider than average variable spread. Common with market-maker brokers.
Variable spreads reflect actual market liquidity conditions. Very tight during liquid sessions (EUR/USD at 0.1-0.3 pips on ECN) but widen during major news releases (5-50 pips or more).
Impact on EA performance
Spread cost compounds across all trades. An EA that trades 100 times per month at 1.5 pip average spread pays 150 pips in transaction costs. At 1.0 pip, it pays 100 pips. The 50-pip monthly difference can significantly impact a scalping EA net return.
For high-frequency EAs, the choice of broker spread matters more than minor optimisation of entry/exit logic.