If it only lags while a lot of stuff on the screen is moving, it seems to me that the bitrate may be too high. Lower the video bitrate and/or resolution and try using VBR instead of CBR, which will automatically adjust the bitrate for you if the encoding is too slow. Also fiddle with the CPU Usage Preset. The faster the CPU Usage Preset, the more bitrate will be required to make it look good. veryfast still produces good looking video for the bitrate though, in my opinion.
If it is available for you, use a hardware encoder (Nvidia's NVENC, AMD VCE or Intel's QuickSync), it will take a lot of the weight off your CPU.
