it's not decrease, you just play in low res on Full Screen that's why it's smoother, while in borderless mode actually you will have v-sync enabled from windows itself, if you play in Full Screen [imagine there's no problem] you will have same performance as borderless mode with the same image quality, the downside is, in Full Screen you will have screen tearing and annoying 1s blackscreen when you change to other windows [or going back to desktop screen]

so for some reason you're not getting 1080 on Full Screen, but you're getting 1080 in borderless, borderless is better, you should go borderless even after fixing your Full Screen issue.

GL