Here we can observe confirmations from multiple users:
"45 seconds to start up Supermium..."
https://github.com/win32ss/supermium/issues/358
Like I said, this doesn't happen to me with Ungooggled.
For me, it's not 45 seconds, but more like several seconds vs milliseconds in Ungoogled, still very high, especially if we consider the fact Supermium "greets" me with a blank white screen (despite the dark theme currently forced on), and it stays unresponsive for like 7-10 seconds. With extensions it'd be much, much longer.