What I start early are greens--lettuces and Asian mustards. This year it was so warm in February, that I set up a little tent in the garden, just a couple of feet high in the center, with translucent plastic from the hardware store over metal hoops. The lettuces I sowed are big enough to get salads from the thinnings. In March I did the same with the mustards, which grow faster, and they're big enough for thinnings to add to the salads as well. I also sow Gailan (Kailan) the same way--just up now.

I think your analysis is sound, but it will also depend on where you are and how long your growing season is. The longer the season the more forgiving the climate is for late starters.

My main goal is to get harvest earlier - and not jsut a couple days. Actually revise that to harvest sooner enough to justify the effort - at least a week.

Early season transplanting and protection of lettuce, spinach, and kale have worked to get fresh garden produce on my table sooner. Overwintering them, with protection, gives an even earlier harvest.

The heat lovers such as tomatoes, squash, etc. don't respond as well. Containers can give an earlier harvest for some of them, but they are more work than in ground. It's usually worth it to me to do a large pot or two of an early tomato, and maybe an herb planter, but I seldom fuss with more than that.

I start a few early compact tomatoes in mid Feb. These go into 3 gallon pots (or smaller) and I can put them outside on lovely days and easily bring them in at night time. I usually get riper fruit in mid-June. The rest get started at the end of March for plant-out on May 21 or June 1, depending on the weather.

I always start tomatoes and peppers early but everything else is direct sown. Lettuce. radish. peas and onion sets like cooler temperatures so they are planted when it's still cool but the soil is slightly warm. Everything else waits until it is close to frost free or frost free and the soil is warm. If something takes a long time to germinate it goes in first. Things that only take a week to germinate goes in when it's frost free. As long as the seed is under the ground it won't freeze but if you plant too early there's a danger that the seed will rot. If something comes up and there's a danger of a couple degrees of frost I'll cover them with a row cover and leave it on until all danger of frost is gone. Row covers protect with a couple of degrees of frost. They allow water to go through and 80% of sunlight. I do set some up side down pots along the row because water can be heavy before it seeps through. Tomatoes and peppers are the last to go in the garden.

I did an experiment once with cucumbers. I started some in my greenhouse and some direct sown in the garden. In 3 weeks both were close to the same size and both produced fruit at about the same time. I decided it was a waste of time to start them early

I sow salad greens and green peas under a spun fabric low tunnel so that I can grow and harvest most of the winter. I start tomatoes and peppers under lights two months or more prior to planting out; everything else (the Cucurbits and beans) gets sown directly in the garden when it's 'time'.

My garden is small but productive and the fabric makes a huge difference without a doubt. I don't use anything on top of the soil other than a layer of mildly composted wood chips+leaf litter.

Dowlinggram, how interesting about the cukes. I could save a lot of time and effort not making special newspaper pots and growing my cukes early.

Labradors. try your own experiment and see what results you get. It may be a different growing zone will produce different results. My conclusion from my experiment was that it takes a couple of weeks for transplants to settle in and sink their roots in the soil and become accustomed to the changes in their environment. By then the seed planted ones had sprouted and started to grow.

All I know is that for me it's not worth the bother to start cukes early

I am with dowlinggram lots of work, lots of seeds do not germinate, and almost no earlier harvest and overall harvest nearly identical. I did notice a melon seed that accidentally germinated inside a larger heated cold frame I built started fast and early. It never did anything (I cannot remember what happened to it). That said I am going to try to start a watermelon inside it again this year - at the very end so it simply grows out - I can gt the seed in the ground 2 weeks earlier. In my zone that makes a big difference.