Installing 2 x 16Gb and 2 x 32Gb on an iMac 27"
In general, for any Mac that’s not core 2 duo, it is hard to benchmark any speed difference no matter how you choose to install ram. (Let alone see it in practice)
Apple publishes manuals for each specific Mac, so let’s see what if your hardware has any recommendations:

https://support.apple.com/manuals
https://support.apple.com/en-us/HT201191 (Memory specific info)
How should different sized RAM be added to iMac?

The 2019 model doesn’t say anything about order like the 2020 iMac does which says to add up the A channel and B channel and minimize the difference between the two.