Focus is based on the contrast in the picture. When you use spot metering, you limit the contrast and lose or limit the focus. You'll see this when photographing birds against the sky especially when the sky and the bird are close in color.
Try evaluative metering.And what benefit would that give? I'm not too sure that PDAF has anything to do with metering.
Yes, focus is based on contrast...in the scene (PDAF depends on contrast to generate the phase difference). Metering affects exposure which affects the contrast in the captured image only
. Metering information is used to determine aperture, shutter speed, and/or ISO, depending on your settings. Metering has no effect
on autofocus (at least, not in the user's T1i/500D; metering data does contribute to face recognition/tracking in the 1D X, and allows compensation for light source, e.g. fluorescent, with dual-layer iFCL metering - but even then, metering type - spot vs. evaluative - doesn't matter).
Light coming through the lens hits the reflex mirror where most of it is directed upward to the viewfinder and metering sensor, and some light passes through the reflex mirror to the secondary mirror, where it's reflected downward to the AF sensor. So, the light hitting the AF sensor never reaches the shutter or the imaging sensor - that means settings for shutter speed and ISO are irrelevant. AF is performed with the lens aperture wide open (it's stopped down to the selected aperture, if necessary, just before the shutter opens), so the selected aperture also has no effect on AF.
So...what effect do you think spot vs. evaluative metering is having on AF? How would that work in fully manual mode (M, user-selected ISO), where metering does nothing except move the little arrow up or down on the light meter display?