Hello Varun,
The new URL scheme is there because we took the opportunity of our latest data update to rework a bit our infrastructure holding these data.
This data update is only available through our currently in-developpement web-mapviewer, that is bound to replace
map.geo.admin.ch very soon. But that meant that we had to keep the "old" data available while publishing the new (transition period). The data from the first URL you gave is meant to be used with a (now pretty) old version of Cesium, and isn't 100% compatible with the latest version (that's one of the main reason we are updating our data, we want to use the latest version of Cesium on our ongoing development)
So the caveat is that the first terrain URL scheme you mentioned might be decommissioned in the future. As soon as we have finalized our move to the new codebase we might start removing the specific stuff that was there for the mf-geoadmin3 viewer.
Regarding speed of loading/performance, we haven't done too much performance refinement yet, so there might be some leeway to improve load/performance in this regard.
Hope this answers most of your questions,
Best regards,
Pascal