TVS diodes (ESD protection diodes) are usually placed between the signal line and GND. Therefore, in steady-state operation, the diode appears as a capacitance (terminal-to-terminal capacitance). These capacitances are included as impedances depending on the signal speed (particularly high-speed-signal LVDS etc.), resulting in signal loss (inserted power loss: Insertion Loss) and degraded signal quality.
The larger the capacitance Ct between the terminals, the larger the inserted power loss (the numerical value is prone to significant negative fluctuation) and the signal speed cannot be followed. In order to prevent such signal degradation, it is necessary to select a TVS diode with a capacitance between terminals suitable for the signal speed.