If any one part of your body is having to apply a lot of force to the guitar, your shape and position changing is going to be slow. That's because however strong your are, it is always going to take time to release pressure, move to another position, and put pressure on again. That's also a recipe for strain injuries.
Let your whole upper body share the work. Move your left elbow around so that your fingers can get in roughly the right shape for a chord without bending your wrist or putting your fingers in too unnatural a position. Let your fingers focus on getting in exactly the right shape for holding the chord, rather than pressing hard. If you're pressing hard, not only are you going to slow yourself down when moving to the next chord - you'll also find it hard to move a finger or two when you're trying to do chord-melody style riffs. Where possible/necessary, you can use a bit of right arm pressure on the guitar body to take some of the strain off your thumb.
Make sure your guitar is doing its job too. An awful lot of instruments come from the shop set up in a way that an F barre chord (for example) takes too much pressure. Make sure your action is low enough for the techniques you're trying to use. Also, make sure your guitar is high enough and stable enough to be able to adopt all the positions you need comfortably. You shouldn't be needing to hold the instrument up with your left hand.
Push yourself to improve, but not too fast. It always takes time to build up strength, suppleness, and most importantly the finger independence and precision that lets you get the fingers in the right position (and the experience that tells you, when you do have a slight buzz, which finger you need to move, and which way!)
Cheat. You don't actually have to get all your fingers down in exactly the right place at the same time. Get the important melody or bass note down first, play that, and then you can play the rest on the next beat or strum. (obviously this is more applicable to some styles than others - 'miss a few notes' won't wash if you're playing a classical guitar piece from a score!)
So, I haven't talked about the thumb much have I? The only generally-applicable thing I can say is, make sure it's working as part of 'the team'. Sometimes you may want only the tip of your thumb on the back of the neck for maximum range of individual finger movement; Other times, it can relax and lie along the back of the neck as a guide, with you only needing to push gently with your palm; sometimes, you even need it wrapped round the front to assist with muting (this is admittedly more relevant to bass playing, in situations where you can't use right hand muting, but comes into electric guitar playing too). And as others have commented it also depends on the instrument - cross-section of neck, width of fretboard, and so on.
Every position will make some things easy, and other things harder. Whatever you want to play, get into the body position that makes it easiest, and if you're doing it right it shouldn't feel like you're pressing very much at all.