This is one of the reasons I've started building https://formpress.org. Seeing the bloat in many form builder apps/services, I've decided there is need for a lightweight and open source alternative.
How we achieve lightweightness? Currently our only sin is, our inclusion of jquery, that is just to have some cross browser way of interacting with DOM, then we hand craft required JS code based on features used in the form builder. We then ship a lightweight runtime, whose whole purpose is to load necessary JS code pieces to have a functional form that is lightning fast. Ps: we havent gone to the last mile in optimizations, but we definteley will. Even with current state, it is the most lightweight form builder out there.
It is open source, MIT licensed, built on modern stack(react, node.js, Kubernetes and Google Cloud) and we are also hosting a freemium version.
I think, there will be ever increasing need and market for lightweight products, as modern IT means a lot of products coming together. So each one should minimize their overhead.
Give our product a go and let us know what you think?