I have a plugin that performs some searches and returns a number, I would like to wrap that call to the plugin inside a promise to improve performance in a client script

Wrapping synchronous code inside a Promise does not make it faster

no but it should free up the rest of the client script to continue while that slow function is being executed, no?

It wont

so the only way to get what I am trying to achieve is to rewrite the whole function to do the searches using promises?

Yes