If started indoors they can be very spindly. However, they're not like that in my unheated 6'x5'x3' Strong Camel greenhouse. They come up quite short and squat.
They're just as easy to start as tomatoes in there, but they do start out smaller. I think they just need more light and maybe cooler night-time temperatures to avoid being spindly. They certainly get that in the greenhouse.
If you want the overwintered ones to germinate faster, try tilling and watering the soil where they are. Tomatillos have a *lot* of seeds (albeit very small seeds). Even if you miss a few fruits, they can be growing like weeds all over where they were.
I haven't noticed a big difference in growth rates between varieties, but I've only grown about 8 varieties or so. They seem to cross-pollinate easily.
I like Cisineros for flavor and size, so far, although a kind I got from my friend can taste totally awesome and sweet when fully ripe when they fall off the plant (although it can also have a strong potato taste/smell at other points and then, too, which is interesting). Amarylla tasted like sour watermelon to me. I've also grown the generic one from Trade Winds Fruit, the Yellow one from Trade Winds Fruit, Mexican Strain, and Purple De Milpa (which didn't get fruit ripe enough to eat, although it did get a big, flowery plant, and it seems to have cross-pollinated my friend's tomatillo, which cross I count as the eighth).
This year, I'm growing a few more, and not all of those above. The standouts I've grown in the past were Cisineros, my friend's, my friend's cross, and maybe Yellow.
I highly suggest transplanting before it gets too hot and dry.