I noticed your outline width is 4; however, the overall visual effect is that it looks like a single pixel. If your text (and graphics) are all scaled up to look that way, you might consider rendering unscaled to a render target the quarter of the size of your full screen. This will drastically reduce the amount of pixels your shader has to process and will also allow you to set your outline width to 1.
Sorry if I misread how this is working 