Is it as simple as that? You just press the child pays for parent and electrum does the correct calculation to confirm quickly?
There is no "Child Pays For Parent" button, and no, it is not as simple as that.
The receiver needs to spend the unconfirmed coins in a new transaction, ideally of the smallest possible size. The easiest way to do this is to send all the coins to a new address he owns. Such a 1-input-1-output transaction will be somewhere in the region of 144 to 192 vbytes, depending on address type. He would then have to add the size of his transaction to the size of the unconfirmed transaction, to get a combined size. He would then have to select a fee rate which could confirm quickly, work out the total fee required to cover that fee rate for the size of both transactions combined, subtract the fee you have already paid, and then pay the remaining fee. For example:
Lets say the unconfirmed transaction from you to him is 300 vbytes large and has paid a fee rate of 50 sats/vbyte, for a total fee of 15,000 sats.
Lets say he makes a new transaction which is 150 vbytes in size.
The combined size of both transactions is therefore 300+150 = 450 vbytes.
To get near the tip of the mempool, lets say you need a fee rate of 200 sats/vbyte.
This means the combined total fee will need to be 450*200 = 90,000 sats.
You have already paid 15,000 sats, so he will need to pay an additional 75,000 sats.
His transaction is 150 vbytes, so 75,000 sats would work out as 500 sats/vbyte.
Bitfinex do.
Huh. TIL. I don't put much faith in Bitfinex given the whole Tether shadiness, but good on them for actually keeping up to date with bitcoin developments.