You’ve been doing that for 15 years. Cut me a little slack on trying to do it for five minutes.
Well, I understand the meaning of the second phrase, something like 'easier, I'm learning it for only five minutes', but I don't understand the construction of it. Is 'cut somebody a slack' an idiom?

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Answer

'To cut someone a little slack' means to go easy on them, to be less demanding or critical or to make allowances for them. The idiom originates from tailoring something to make movement easier.

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When elastic is slack, it is not stretched, it has no tension.
The meaning is extended to ask someone to stop asking expectations, or stop creating tension.
I think you can figure out the rest from here.

Answer

Yes,it is an idiom.
'cut someone a slack' means 'give someone a break'