MCU Performance: running with emulator vs running without emulator
In a word, yes. 

As a rule of thumb, a debugger will slow down the target chip. The more expensive real time debugger/ICE tools reduce this but you will still get a measurable slow down. 

The slow down is typically because the debugger sticks extra code into your program for breakpoints, RAM monitoring etc.

How much slow down is rather a "how long is a piece of string" question. Your best bet is to measure it.

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Word to the wise

Always test your code comprehensively without a debugger. It is entirely possible to write embedded code that only works when the debugger is plugged in (the debugger slow down fixes timing issues inadvertently).