Just out of curiosity, and because other members might find also it interesting:

Does the typical commercial passenger jet of a given model have a standard cruising speed which it maintains by default unless there's some reason to go faster or slower? Or to put it another way, how much leeway does the plane have to adjust its speed on a given flight segment up or down from the speed it usually maintains, depending on things like weather conditions or whether it's ahead of schedule or behind?

I'm sure some knowledgeable people on this list could comment. Thanks.