After creating a portable folder within the IDE installation folder on C drive the IDE doesn't boot at all. You see the IDE pops open for a moment and then disappears.
You must have the Arduino IDE installed to the Program Files (x86) folder.
The correct place to create a working folder in Windows is NOT under the .exe directory, but in the AppData folder for the user.
I'm sorry to hear it didn't result in any improvement. Well at least the negative result tells us that accessing external folders is not the cause of the slowness. Unfortunately I'm out of ideas. The antivirus and the external folders are the only two potential causes of slowness I could think of. Hopefully someone else here will have another idea.
Great news! Now you could stick with portable mode or you can investigate further to try to find out how you can resolve the slowness issue when in non-portable mode. It's up to you....So the next step in the investigation would be to determine which of those two folders were the cause of the slow down....After doing that, there will likely be two possible results: If the Arduino IDE is still fast then you know that the problem is with the location of the data folder when not in portable mode. If the Arduino IDE is slow then you know the problem is with the location of the sketchbook folder you set in File > Preferences > Sketchbook location.