I think the interest rate on cash advances obtained via credit card will usually be much higher than what you lose at a currency-exchange booth. Google tells me the interest rate for cash advances on my United MileagePlus card was over 20% as of 2024 (an increase from the 2023 rate). Although rates at airport exchange booths are notoriously bad, I don't think I've ever seen an in-town rate as bad 20% off the interbank rate.
But you are comparing an interest rate vs a conversion rate, two very different things.
The conversion rate is a percent based fee, so if the transaction is $200, at a 20% conversion fee, your cost will be $40, there is no time element.
Interest is typically expressed as an APR, an annual rate. You would pay 20% of the transaction, if you did not pay for a year (actually more with compounded interest). But if you, as many on here say, pay off your credit card monthly, and you carried that $200 the full month, at 20-24%, the interest for the month is around 2%, or about $4 on that $200. Of course, after you withdraw cash, you could go online and pay your account off immediately, avoiding much interest.
There is also a fee associated with a cash advance. usually a minimum, $10 or 3-5% of the transaction if that totals more than $10.
So a $200 cash advance, for most card holders, would cost a $10 fee, plus up to $4 in interest, or $214. That is a 7% markup, comparable to getting cash at a bank in the US, Hard to compare to exchange places in Europe (too much variability and shady places), and certainly less than airport counters at around 10-15%, or more.
My UNFCU Elite card offer cash advance for $5 or 2% of transaction, so I can get a cash advance of $200 for less than 5%. Just never have been in a situation where I needed to use it.