Hmmmm. Mainspring possible but not likely. Could also be the hammer, or the trigger/sear/disconnector spring. Will take some fiddling, but you might leave the sear out and see how stout the spring is when returning only the trigger. Then add sear and see the difference. Easy to tweak the three-fingered spring, but it takes tiny bits of tweaking and trial and error. Midway has good "tuner" reduced power trigger/sear/disconnector springs for 6.95 to $8.00 or so.
There are hammer/sear combos available that will not break the bank. To mess around with, a couple months back, I picked up an RIA keyhole hammer with strut, sear and disconnector for $15 plus S&H. At that price, they can either be smoothed up, used as is or tossed. Current "Sale" price at AdvancedTactical is 29.99.
Lotsa tips on the 1911 forums, but all say to keep trigger at 4lbs or greater. Still, that's half of what you have and will feel great.