There is a delay between weapons for the most part. Try switching to a grenade from your rifle, priming it and throwing it, especially when walking or running. It can seem to take forever. As for rifle to pistol, it's pretty fast, but it does not take that long in real life to pull a pistol from a holster and flip your rifle around on your back at the same time (remember you are not just holding it, but it is attached with a strap).
I just stopped the weapon switching time from a some weapons:
It takes about 0,9 sec to switch from whatever weapon to MP44
and further:
0,9 sec for Pistols
0,8 sec for Russian PPSh41
0,7 sec to F1 Grenade
1,7 sec for Stiehlhandgranate
1,2 sec for PAK
These values are not 100% accurate but they are pretty close.
Of course you need further iron sights for rifles and PAKs for accurate shooting. Also you have to cook and throw grenades.
But still within this time you shouldered your actual weapon and switched to another one. In case of rifles, smgs and pistols you can immideately use them. And they are already in a perfect position for accurate hipshooting.
A well it doesn't matter what you are actual doing like running, standing or lieing.
As for turning your body...just stand up and turn fast, the game is almost too slow. In real life, you can turn and face multiple directions and be aware of more of your surroundings MUCH faster than in game, and I don't know about you, but for me, when my body turns and stops I am not carried continually around by my own momentum. If the human body could not overcome the momentum of it's own movement that would really put a damper on sports like Soccer or Football or Hockey or Tennis.
True for only moving your body.
But if you use a weapon you must after rotating or turning balance it out for accurate shooting. And then hold a 5kg heavy unhandy rifle in your hand and repeat that. It wouldn't probably take very long, but it isn't a instant thing like it is in RO.