Instant execution versus market execution

[Shadow Trader]

Two execution models show up in broker specs, and they feel completely different in use.

Instant execution: the platform shows a price, the order requests that price, and the broker fills at it or sends a requote. In calm markets, that feels perfect. In fast ones, requotes pile up, and the trade the setup depended on never happens.

Market execution: the order is sent without a price promise, and it fills at whatever is available. There are no requotes, but there's slippage in both directions.

On the flip side, instant execution can feel safer in quiet markets, since the price is what the screen showed.

The execution type is a property of the account and sometimes the symbol, so it can differ between pairs on the same broker. A quick look at the contract spec shows which is in use.

Does execution type affect how a strategy performs more than the spread does?