Segwit is only creating an extented block (the segrated witness) to remove data (transactions signatures) from the orignal block to free some space. It has nothing to do with the speed of a transaction. Dash Instasend use the masternode network to validate 6 confirmations presto (in a matter of seconds) based on a quorum.
Frankly, segwit is a weird tweak to increase the blockchain size without increasing the 1MB minable blocksize.