Stiff - The inability to bend strings and have that smooth, silky, 'buttery' feel.
How stiff your guitar plays depends on a few factors. The gauge and brand strings you use, the number of springs you have on, the angle of the trem, the height of the action, and the contact between the strings and frets.
I've been using DiAddario XL120's for years and what I use on all used guitars. For a 7 string I use a .054 on the bottom. I just happen to like the feel of them.
4 springs will always feel more stiff than 3. Even though they exert the same tension they do it from a more closed position. The more closed a spring is, the more force it takes to get it open. 2 springs in arrow formation will give you the spongy-est feel as they are the most 'open' of any configuration.
One of the reasons many people love the feel of older guitars is how spongy they feel when bending. This can be directly attributed to old and near worn out springs. Eventually they will need replacing but for the long period when they are weakening [but still useable] they are just buttery.
I've always found the springs get stiff from lack of use, like
they're static. They need to be used, and used a lot to get spongy. Springs react
weird to lots of things. Getting tweaked too much doing a setup they get *confused* until
they finally figure out what you're asking them to do! And then don't fully settle in for
24 hours. Not being stretched at all for long periods makes them really stiff until they
get abused a little to soften them up again. So abuse them!! :-)