While that comment about making you want to buy DA is funny...well it's not true! LOL!!! All you have to do is switch to 105 and you will get about 98% of the feel and the same stopping power of the DA, while the 105 will weigh a bit more but not a lot more than DA, I think it's around 120 grams difference, which is around 4 ounces. I was able to actually improve the feel, ever so slightly, with my 105 brakes by simply changing the original Shimano standard cables to DA9000 cables, with that change I can't tell the difference at all between my 105's and a friend's DA's, and neither could he. So I think the reason people say that DA is smoother is that bikes that come with DA components come with the better DA9000 cables!
I test rode a lot of bikes back in 2012 and 2013 due to wanting to buy a new bike, and I can tell you that there is very little difference in the feel of 105 vs Ultegra or DA; my wife had Tiagra brakes on her bike and there is a real perceivable difference in feel I get immediately when riding her bike to test something that needs adjusting. But how much worse is that feel? it's probably around 80% of what the others are, so maybe a 15% difference, not that much really.
Now pay attention, most people don't understand what I'm about to say. Tiagra, 105, Ultegra, and DA ALL have the same stopping power! Why is that you scream? because the stopping ability is all about the tire's adhesion to the pavement, once you reached the limits of tire adhesion it doesn't matter what type of brakes you have EVEN DISK brakes you will stop the same with aluminum wheels as rim brake if there is no rain present! What you are feeling in the Tiagra is the more actuation and hand strength it takes to get the brakes to work to that point.
Having said all of that, without test riding your bike I don't know if your Tiagra brakes are any different than my wife's bike, if yours are harder to operate it's probably not because of the spring. The issue is probably either the cables need to be replaced and or you need to put a drop of oil on the pivot points on the brake calipers and levers. I doubt the calipers are out of adjustment since you said it affects both brakes, it's highly doubtful both would go out at the same time. What kind of lube should you use? I would use Tri-Flow or something like that, I would not use a wax-based, or a dry lube you might be using on your chain.
If the oiling doesn't work, like I said it could be the cables, but if the cables don't do the job either than the brakes need to be rebuilt...but at that point I would just spend the money and get 105 brake calipers if you want to keep the bike for a long time, the swap will only cost around $130 plus labor, and just keep the Tiagra levers since not much can go wrong with those.
You could try doing this if you're mechanically inclined before dropping the coin for new calipers:
View: https://www.youtube.com/watch?v=wPJRddS_sRo
You could try a set of DA cables, but I don't see the point of it with Tiagra, but it could help, but only maybe a 2 to 3% better actuation. I would first oil all the pivots as I mentioned earlier making sure you work the brakes a bunch of times to get the oil to work its way inside the pivots, this is cheap to do and you can do at home, ride the bike and see if there is any improvement, if so, you may not need cables, but you probably will need cables.
