I'm a firm believer that "organization" and "routine" are key in increasing your speed... but once on the road your priority should be "safety" not speed.
More information on your situation is needed. How long have you been working? Daily volume? LLV or your own vehicle?
I do things differently in that I have a LLV and carry a separate tray with "stand up" flats. I still use the thin flats as "binders" for the letters and "roll" them in... but multiple flats to one addressed get placed inside the thickest one and those along with any thick flats come out of the case before the letters go in.
#1 Upon signing in I immediately case ALL of my flats.
#2 I then work my parcels. Large ones to the side, while a break the route in two sections, with an empty tub on the floor for each. As I pull the smaller parcels out of the bin they go in the respective tubs.
I do this now because some "parcels" can easily stand up with the flats.
#3 I then pull down flats leaving only the thin ones that will hold each boxes letters and stand them up (addresses near top) in a hard plastic tray.
#4 Then I case all of my letters. DPS first, then raw letters. That's because its easier to slide in a handful of dps letters into a near-empty case, as opposed to putting sporotic raw letters first, impeding sorting the multiple dps letters into the same slot.
#5 I fill out any peach slips for accountables and prepare them.... putting them in front of the address (if they're letters (certified).
#6 I use parcel markers... so I mark the parcels as I pull them out of the tubs and place them in piles by rows of the case. I then put the parcels in delivery order in another hard plastic tray.
#7 I take care of my forwards, mis sorted, etc and check the hot case for a final pull and case that mail.
#8 Then it's time to pull down and go.

I get a lot of flats on my route (half businesses) and can typically get them cased and pulled down in the first hour I'm there.
Then I figure 10-15 minutes per tray casing time for the letters... so 6 trays equals 90 minutes. Another half hour to mark parcels and pull down.
Typically about 3--3 1/2 hours in the office.
In my LLV I have 3 hard plastic trays across the truck's tray. a/letters, b/ stand up flats, c/smaller parcels.
There are as many different ways to do this job as there are carriers... and over time each of us finds the "system" that works best for us. A lot of times that "system" you develop comes from observing how other carriers do things and pick out what make the most sense for you.

Rome wasn't built in a day and practice makes perfect.

Don't waste time pressuring yourself. Use that time constructively and believe in yourself. Never stop learning.... and don't give up... RCA's make a damn good hourly wage that you won't find anywhere else without a college degree.

Women have typically outnumbered men in the rural craft, especially in days gone by when housewives were looking for something to do when the kids leave the nest and men weren't available for part time work because of their traditional "breadwinner" and full time job responsibilities.

I'm not sexist... as I said those were in "days gone by".. I'm just trying to point out that MANY women have come before you and have no problem handling this position. In my office alone we have 3 of the most petite women I have ever seen and two of them are regulars.

You can do this job and do it well. And you will get better. But remember, patience is a virtue.

Yah, that's pretty much how I do it too...I write down my packages instead of flagging them though. Every second really DOES count. Every second you waste per box multiplied by 600-800 adds up to lots of minutes and hours, so I always try to have my mail in hand before I even stop at the box, that's why having letters inside the flats is nice. I never could understand why people strap their mail and pick through it all over again on the road. I like to fill out my 3849's in the office too...I write a lot quicker at my desk than in the jeep, and some people are just too creepy to be near that long filling out slips out there...