I had a M1 Super 90 years ago, and it ran sluggish sometimes. I would check and clean/lube the buttstock/recoil spring tube. My M1 always seemed to be a magnet for unburnt shotgun powder, that somehow found it's way into the tube!
Other than that, get a replacement spring and swap it out before you try anything else.
My rifle had no problems shooting the (reduced load) Federal tactical buckshot, slugs or whatever else I fed it. I only sold it out of stupidity, and never replaced it.