const FAST_LOAD_TIME = 500;

const clamp = (num, min, max) => Math.min(Math.max(num, min), max);

function loadAndWait(minimumLoadTime = FAST_LOAD_TIME) {
    const startedLoading = + new Date();

    return function waitMinimum() {
        return new Promise(resolve => {
            const loadDuration = new Date() - startedLoading;
            const waitTime = clamp(minimumLoadTime-loadDuration, 0, minimumLoadTime);
            setTimeout(resolve, waitTime);
        });
    }
}

async function doSomething() {
    const waitMinimum = loadAndWait();
    // Do something that the user expects to take some time, e.g. loading data ...
    await waitMinimum();
}