YoshiGo99, it depends on Phone.
Certain rooted Android Phones and Tablets can capture direct feed video at full speed, but some of them (Nexus Phones, Tegra-based Tablets/Phones) have trouble with this.
Jailbroken iPhones, iPod Touches, and iPads can do this too, but it's on low Frame Rate.
If supported, you can use an HDMI Plug, an HDMI Capture Device, and capture direct feed video via this instead.