Do you already capture to an SSD?
Code:
18:13:35.426: [x264 encoder: 'simple_h264_recording'] preset: ultrafast
18:13:35.426: [x264 encoder: 'simple_h264_recording'] profile: high
18:13:35.427: [x264 encoder: 'simple_h264_recording'] settings:
18:13:35.427: rate_control: CRF
18:13:35.427: bitrate: 0
18:13:35.427: buffer size: 0
18:13:35.427: crf: 14
18:13:35.427: fps_num: 240
18:13:35.427: fps_den: 1
18:13:35.427: width: 1920
18:13:35.427: height: 1080
18:13:35.427: keyint: 250
The key interval sounds useless to me. You record with 240fps (is ultra slow motion a thing in Minecraft, or for what do you record with such a ridiculous framerate?) and your key frames are set to every 250 frames.
I would set the keyframe interval to 2sec which means "keyint: 480" in this case.
1080p at CRF14 with a crappy preset like "ultrafast" will result produce very big files or in other words your systems needs to push a high amount of Mb/s to your harddisk/ssd, especially with 240fps.
Those lags could be a result of the intense disk activities.