It doesn't lug as much and revs down more slowly ,also shift-up and down seems smoother at least until the bike is really warm. It's always been this way. I don't ride with with the choke that way for fear of running too rich. Anyone know why?
thnx.

I'd agree with T Mack.  When temps get colder and the air gets drier, it has the effect of making your fuel mix much leaner that it would have been in  warmer temps and higher humdity in , say, July. By going to half-choke you're richening the mixture to bring it back closer to the ideal air and fuel mixture.

So, the solution would be to richen the mixture during the colder months? My bike took a while to start this morning. Temp was about 31F. After she fired up, she ran fine. I don't like the starter turning over 10 times a day though, may burn it out.

Richening the mixture in colder months will restore the proper  ratio of gasoline to air for optimal combustion efficiency and power.   What happens is that as the air gets colder it becomes denser. Denser air means more oxygen is packed in a unit volume of air, which nets out to a leaner mixture (since the amount of gasoline is unchanged.)

So how does the choke help get the fuel mix back to normal?

Adding some choke results in more vacuum, and this sucks more fuel out of the idle circuit and other jets to richen the mix.  If you only adjust your idle mixture screw a bit richer in the winter you'll have better idle performance but that won't help you at higher speeds. If you add a little bit of choke you'll pull more fuel out of the other jets at higher speeds, which is what the OP observed.

Riders might also notice that their bike backfires more in the colder weather than in warm weather. Same explanation -the mix goes a lot leaner and so it's like a Savage sounds before the idle mix screw mod is done.