Bjorn Nyland has shown a charging process of the Ioniq 38kWh at a 350 kW fast charger from Fastned, near Amsterdam. I took the numbers and the following graph was the result:
Then I made an overlay of a previous graph (adopted from the Kona and used for the 2020 Ioniq in Abetterrouteplanner) to compare them in more detail:
So, they are not exactly the same but still quite comparable.
What can be seen is that up to 68%, the power levels are around 42 kW (up to 50%) and around 36 kW (from 50 to 68%). In total, in 39 minutes you get 68% starting from 0, and this is 26 kWh (what is 93% charge for the 28 kWh Ioniq). If you do not start at 0, but, for example, at 15%, then it will be faster (with an average charging power of 40 kW): 31 minutes, which sounds a bit more bearable than the 54 minutes of the official documents. In these 31 minutes, the 26 kWh you get is 16% more charge than an 80% charge for the Ioniq 28 kWh (which is 22.4 kWh). When subtracting the 15% charge at the start of the charging (and 20% for the old Ioniq to get equal kWh), this 16% extra becomes 20% extra, so you can drive 20% further until you get at 15% again, so after 5 times you can skip one charging session compared to the 28 kWh Ioniq. Plus you can start from home with 36% extra charge.
For the total traveling time, this means that a difference of only a few %% is obtained, often just between 0 and 2% (especially for trips up to 1000 km), as can be checked on Abetterrouteplanner for your favorite trips. For longer trips, it may be 5% difference, but even that can be less; for example, for a 2000 km trip from Amsterdam to the South of France, no difference at all is found on Abetterrouteplanner.
If you want to go above the 68%, to 80%, for example, the charging is really slowing down. You will spend many minutes of these 54 minutes for that phase. You may want to stay away from that phase (unless you are just enjoying your meal).
So, it may be smart to think about a good strategy of charging.