It's electron. You're running a fully fledged Chrome instance(which at this point might as well be another full OS) in order to run a terminal emulator. This also means that it probably comes bogged down with all the shit which infests modern web development [1].
It also appears to take over lots of the autocomplete/history functions from BASH, which also means it will either not play nice with ZSH or carry over that functionality when used with ZSH.
Going over the screenshots there also appear to be a bunch of hand populated options/descriptions(unless it's auto filling based on man) which would take away a lot of the modularity.
Also, it appears to be coded in Node and the Read Me mentions Visual Studio, and REE M$ and all that comes in.