When was it found that it slows down at %75? It seems unbelievable that it would take an extra 80 minutes just to get %25 of juice.
About 10 years ago. Every iPhone (and every other device powered by Lithium technology batteries) reduces the rate of charge as it approaches full charge. The reason? Overcharging it even once will kill the battery, and possibly cause it to catch fire. Virtually every smartphone and tablet on the market uses the same charge regulating chip, so they all work the same way. It's even described in Apple's battery documentation.
However, I know because I've measured charging currents on many devices over the years. It's easy to do; it requires a ~$20 investment in a USB power meter. That's how I know that iPhones from the first through the 5 charged at a 5 watt rate, regardless of the power source. Starting with the 5S Apple increased the charging rate below 75% to 7.5 watts, so if you have a USB power source with more than a 5 watt output (e.g., an iPad USB adapter) your 5S would charge faster with the iPad adapter than it would with the "cube" adapter the phone ships with. Starting with the 7, the initial charge rate is 10 watts.
The best way for you to check the charge rate of the RAVPower is to do a controlled test using it, then using an iPad power adapter (which will definitely fast charge the X). Use the same SoC range for both tests.