Alborz, H3 is not slower than Tenergy when looping. H3, especially the boosted National kind the CNT players are using is awfully fast when looping because of the topsheet. But it is not good or fast for flat hitting. So in essence you have it right that the reason they switch is that the DHS rubbers are not good for smashing. But it is not that they are a slower rubber. They are a different kind of rubber that is not good for flat hitting.
Many players use a HARDER rubber on the FH than on the BH because of the size of the FH stroke (large) compared to the size of the BH stroke (very compact in comparison) and how with a smaller stroke and a softer sponge you get the ball to sink in well enough for the topsheet to really grab the sponge. And with a harder rubber it works better for the larger stroke so that you don't get the ball to sink in too much and bottom out on the big strokes while looping.
But the hard DHS sponges are for those tacky topsheets and they are not great for flat hitting.
So, that is why the CNT players twiddle their rackets for smashing.
And H3 is not slow rubber as long as you are using it the way it is made to be used.