That delay is due to forceclosing and redraw of the stock keyboard that for some reason is connected to Samsung's much maligned SPCM service that also has been blamed for the overly aggressive RAM management on its recent Android phones.
In any case, you can get rid of the delay in the SwiftKey keyboard draw on your Samsung Galaxy S6 (also shown working on the Note 4, and perhaps others with the latest update). Here's what you need to do:
2. Open your build.prop file, and replace the line:
3. The SPCM servce culprit should be disabled now, and your keyboard lag should be cured, along with other nasty RAM management surprises the latest Samsung TouchWiz edition.