User equipment mobility is one of the primary challenges for the design of reliable and efficient wireless links over millimeter-wave and terahertz bands. These high-rate communication systems use directional antennas and therefore have to constantly maintain alignment between transmitter and receiver beams. For terahertz links, envisioned to employ radiation patterns of no more than few degrees wide, not only the macro-scale user mobility (human walking, car driving, etc.) but also the micro-scale mobility - spontaneous shakes and rotations of the device - becomes a severe issue. In this paper, we propose a mathematical framework for the first-order analysis of the effects caused by micro-mobility on the capacity and outage in terahertz communications. The performance of terahertz communications is compared with and without micro-mobility illustrating the difference of up to 1 Tbit/s or 75%. In response to this gap, it is finally shown how the negative effects of the micro-mobility can be partially addressed by a proper adjustment of the terahertz antenna arrays and the period of beam realignment procedure.