Fast charging does damage battery health, this is why many switch to normal/slow charging once you hit 80%.

If you are the kind to have a new phone every 1-3 years, then it’s not gonna matter; but if you are planning to have it for like 6yrs, then only fast charge when needed.

Fast charging is just an alternate way of delivering power through standard 5V USB lines.

Regular USB lines are rated at 5V. To allow transmitting higher power without ramping up the amperage, you add voltage instead.

This creates a problem that the same connector now comes in higher voltages, which older device may not be able to withstand and will just fry if clueless users plug them into the new charger.

So There is a series of protocols to authenticate between the charger and the receiving device before it switches to fast charging, without authentication, the charger only outputs at 5V.

For charging time, OP did not provide enough information, like the model of the phone, charging from what percentage to what percentage and the temperatures. Charging will become slower above 80 and even below 80 if the temperature is out of the safe range.

"Fast charging" is not one specific protocol anymore - it depends on the brand and the specific model of phone, as well as which specific charger you're using. For Samsung for example there is now regular charging at 15W, 'fast charging' at 25W, and 'super fast' which garuntees 25W and can go up to 45W. But I believe a random charger that provides like, 18W, will still say "fast charging" so long as it is getting over 15W, but obviously an 18W charger is still going to charge slower than a 25W charger.

To add on to this, me and some of my friends like to use slow chargers especially to prevent battery damage. And we try to never charge above 90% or discharge under 30%. Li-ion hates heat, fast cycles, deep chargers and discharges.

Some fuel gauge chips require training on a battery (a few charge cycles) to "learn" what the actual state of the battery is. They'll initially be less accurate and get more accurate. I'd guess that is why your charge time went from 40 minutes to 74 minutes if all else were equal after two weeks of use. That isn't sufficient time for your battery to degrade that much.

Another possibility is temperature. There is a good chance your charger IC will throttle maximum charge current depending upon the temperature it measures.