Adobe Premiere Pro CC editing/playback performance
There are a couple factors which can be giving you lagy playback.You don't specify your bit rate or codec. If you are editing a processor intensive codec like h.264 (not a good idea) -the processor could start to be a bottleneck. If you are editing a less compressed format like ProRes your drive or RAM can start to become the bottleneck. Since that is the most likely, I'll address that: 

Because you have only one hard drive; Premiere is running off it, the OS is using it, preview files are trying to run off it at the same time. Easiest thing to try first is adding a hard drive or two. Ideally: Drive#1: OS/PPro, Drive#2: Project/Source Footage, Drive#3: Previews/Final Exports. If need be: put your Project and Source footage on an external drive and put your Previews on the local (fastest) drive. Once your previews are created, the project won't need to access the project and source very much. Use an "uncompressed" codec like ProRes or DNxHD for editing -a good SSD should be able to handle several streams at a time.

16Gb may be enough RAM, but it's not a given. It depends on the number of streams, bit-rate, size, length of the timeline, etc..