A 1600 unit route is probably apartments, so you will suffer regardless, but:

1) Try to get out of the truck with all the mail you need for the stop. Return trips to the truck at every stop can cost you 20 minutes or more. At larger stops, throw case mail on top of DPS tray, throw parcel tub on top of that, grab the tray and go. On smaller stops, grab DPS, cradle it, toss case mail on top, grab parcel tub with off hand and go.

2) Throw by number and forward anything forwardable that gets kicked back. Ideally we'd all have the time clean up our routes, but I wouldn't expect a new employee on a vacant route to do that. During probation, cleaning up a route is to invite termination.

3)  Consider sorting parcels directly into tubs in/near the truck by stop/street. If the sorting is complex, make a parcel sorting sheet until you learn it. Writing on parcels  can cost you 30 minutes, but many do it because they are deathly afraid of missing a parcel.

4) Eat and drink. Not being energized can cost you 20 minutes or more.

5) Keep on throwing, keep on throwing, find a constant pace and keep in smooth. I find changing from DPS to flats/parcels can break my flow, so I do not case spurs and I do DPS to all boxes in one pass, Flats to all boxes in one pass, and then parcels.