I'm wondering the same, i have done a comparison with the same mp3 song and while in Studio One it takes 18 seconds to generate the peaks(it writes a wav file in that time), in Reaper it takes 2 seconds and also in Reaper i'm able to use all the DAW functionalities during the process while in Studio One the DAW is locked until finishing the import, then i tried with an old and abandonware like energy xt v3 and it imported the file instantly. All of the DAWs created a wav file from the mp3, so i think the slowdown in Studio One is related to the conversion process and has nothing to do with disk write speed. I tried with a 2 hours mp3 file to see what happens during the import process, i monitored the project location where the wav for the imported mp3 has been created, it took more than 13 minutes and wav file size increased until the import process ended, so i can confirm that the slowness is related to the conversion algorithm, can't say anything more as all the DAWs i used for the test are based on the LAME software, but for sure something is not well on Studio One MP3 import.
Hope Presonus will look into this, because i often work with big mp3 recordings of my dj sets and at the moment it's faster to open the file inside Reaper and export the wav to Studio One than importing it straight into it.
