The Element units are Thunderbolt 1 chipset, while Ensemble Thunderbolt and Symphony MkII are Thunderbolt 2 chipset.
It is important to clarify what the "speed" designation really means, as it can be a bit misleading.
You can see from this link:
Thunderbolt 2 is an update to the original Thunderbolt specification and takes the original’s two 10 Gbps bi-directional channels and combines them into a single 20 Gbps bi-directional channel. The amount of data able to go through a Thunderbolt connection hasn’t increased, but the throughput of a single channel has been doubled.
Because the Element Series are a lower channel count device that only needs 1 lane of bandwidth, making them Thunderbolt 2 would have no effect but would make them cost the user more. It would not lower latency, only make the Element Series able to pass more channels, which are not present.
On the Ensemble Thunderbolt and Symphony I/O MkII it is much more important to have Thunderbolt 2, because those units are passing far more channels, up to 64 channels with two Symphony I/O MkII units in multi-unit mode.
Thus you will find exactly the same roundtrip latency time on Element as Ensemble Thunderbolt, regardless of Thunderbolt 1 vs 2.