As mentioned here , I found out that in my case it is also caused by high mouse polling rate. If I select 125 polling rate, the play mode is butter smooth. If I select 1000 polling rate, then my FPS in play mode go from 60 to 30 while LMB is pressed down in an instant.
I have these issues since Unity 2017.4, or at least that is the first time I noticed these issues.
My mouse is ‘Logitech G203’, but the issue was happening with my previous mouse ‘Gamdias Hades’ as well. So the manufacturer/model should be of no significance.