We evaluate the performance of Gaussian Modulated Coherent State (GMCS) Continuous-Variable QuantumKey Distribution (CV-QKD), in terms of secure key rate, at different detector bandwidths. We illustrate theadvantages and limitations of high-speed CV-QKD systems using a general noise model in which we considervarious noise sources and their dependence upon detection bandwidth. Experimentally, the feasibility of highspeed CV-QKD is demonstrated by using a GHz bandwidth balanced homodyne detector (BHD) establishedwith commercially available components. We provide secure key rates of transmitted and local local oscillatorschemes under various clock rates.