FastContact for KaiOS 2.x, for one. Written without any "framework" BS and, unlike the stock phonebook (written with React), doesn't lag on any amount of contacts. The slowest part actually is interacting with KaiOS API to fetch/update the contact list in the system DB. All libraries are custom-written.
Discussion
Is that running in some form of webview? A phonebook doesn't sound very complex though and possibly doesn't even need reactivity since it's just local API calls?
As an example, something as simple as a scalable pie chart that moves labels around based on screen size would take me a whole day to create from scratch, and then another day to optimize payload size. But I just implemented it in 10 mins with a library and got all that for "free".
There are 100s of little things like that which is why I use npm etc even though it's a shitshow.