I have also noticed that fee per vbyte decreased a little bit since few time.
Right.
Now miners are including transactions with the fee rate of around 120 sat/vbyte in the blockchain. A few hours ago, the required fee rate for getting confirmation was more than 200 sat/vbyte.
Or maybe my cpfp transaction with 142 satoshi per vbyte succeeded help push the parent transaction. Unfortunately it costs me more than 10$ for 75$ transaction.
The
CPFP transaction was made with the fee rate of 242 sat/vbyte.
It increased the effective fee rate to 123 sat/vbyte and that was enough for the transaction getting confirmed.