How can I compare my time on the current route with the previous time on this route in real time to see if I'm ahead or behind ?

If the previous activity is converted to a course, then you can race against it.

Yes of course, but during activity, I don't know if I'm faster or slower. I have to finish the activity to compare times.

Wouldn't the Virtual Partner show your position compared to previous ride?

Time Ahead/Behind and Distance Ahead/Behind data fields may give you what you need when following a course.

Thanks, but where can I find these fields ? I've already looked everywhere but I didn't find these kind of data fields.

I looked at the list of available fields for the Explore 2 and I don't see those fields. I also checked and the unit does not have the Virtual partner feature. Unless these fields were added in a firmware update, I think there is no solution.

Tap the bottom right button on the home page, go to Training > Race an Activity > Ride History.

https://www8.garmin.com/manuals/webhelp/GUID-E67DE53E-5B08-4903-9767-F58973BC1829/EN-US/GUID-84B51468-4888-4374-BC94-09B18199CE11.html

That's what I'm already doing but I don't know if I'm faster or slower during the activity.

That's because the Explore 2 doesn't have a Virtual partner feature. The Explore 2 is a basic computer without all the training features of the higher end Edges.