The cat-5 cable is electrically a transmission line. A transmission line has a property called the characteristic impedance, which is a ratio of the Inductance and Capacitance, both of which are a function of cable geometry. A transmission line also has resistance, which affects the signal and its propagation.
As a signal travels down a transmission line, it continues to degrade in amplitude as well as rotate in phase. When the signal hits a change in impedance, part of the wave is passed through and part of it is reflected back. The amount that is passed versus rejected is determined by the mismatch in impedance from the cable. The reflected signal can interfere with the other signal data traveling down the line. In an ideal sense, a perfectly terminated line will deliver maximum signal into the load and reflect no signal back. A significant enough mismatch or a high enough resistance can also degrade the signal amplitude sufficiently to render it unrecoverable at the receiver.
With a splice you potentially introduce an impedance mismatch, which you want to avoid. The effective overall data rate will be impacted by the signal quality. Hence, it is possible that your splice will effect your speed. However, with most digital transmission, there is a point at which if the signal is "good enough" you will maintain maximum speed and avoid any data loss. This is an example of why things like heavy gold plated cables for your digital TV signals don't necessarilly buy you any improvement.