Cat5 can handle gigabit speeds, but generally is not rated for that. Is your internet plan greater than 100MBps? If yes, then perhaps upgrading to Cat5e or Cat6 on that one cable may help but I doubt it will really make a difference. If your internet plan is less than 100 Mbps then your Cat5 should handle that throughput with no problem so that isn't a problem or source of any bottleneck.