React renderToString() Performance and Caching React Components

Question

I've noticed that the reactDOM.renderToString() method starts to slow down significantly when rendering a large component tree on the server.

Background

A bit of background. The system is a fully isomorphic stack. The highest level App component renders templates, pages, dom elements, and more components. Looking in the react code, I found it renders ~1500 components (this is inclusive of any simple dom tag that gets treated as a simple component, <p>this is a react component</p>.

In development, rendering ~1500 components takes ~200-300ms. By removing some components I was able to get ~1200 components to render in ~175-225ms.

In production, renderToString on ~1500 components takes around ~50-200ms.

The time does appear to be linear. No one component is slow, rather it is the sum of many.

Problem

This creates some problems on the server. The lengthy method results in long server response times. The TTFB is a lot higher than it should be. With api calls and business logic the response should be 250ms, but with a 250ms renderToString it is doubled! Bad for SEO and users. Also, being a synchronous method, renderToString() can block the node server and backup subsequent requests (this could be solved by using 2 separate node servers: 1 as a web server, and 1 as a service to solely render react).

Attempts

Ideally, it would take 5-50ms to renderToString in production. I've been working on some ideas, but I'm not exactly sure what the best approach would be.

Idea 1: Caching components

Any component that is marked as 'static' could be cached. By keeping a cache with the rendered markup, the renderToString() could check the cache before rendering. If it finds a component, it automatically grabs the string. Doing this at a high level component would save all the nested children component's mounting. You would have to replace the cached component markup's react rootID with the current rootID.

Idea 2: Marking components as simple/dumb

By defining a component as 'simple', react should be able to skip all the lifecycle methods when rendering. React already does this for the core react dom components (<p/>, <h1/>, etc). Would be nice to extend custom components to use the same optimization.

Idea 3: Skip components on server-side render

Components that do not need to be returned by the server (no SEO value) could simply be skipped on the server. Once the client loads, set a clientLoaded flag to true and pass it down to enforce a re-render.

Closing and other attempts

The only solution I've implemented thus far is to reduce the number of components that are rendered on the server.

Some projects we're looking at include:

• React-dom-stream (still working on implementing this for a test)
• Babel inline elements (seems like this is along the lines of Idea 2)

Has anybody faced similar issues? What have you been able to do? Thanks.

Answer 1

Using react-router1.0 and react0.14, we were mistakenly serializing our flux object multiple times.

RoutingContext will call createElement for every template in your react-router routes. This allows you to inject whatever props you want. We also use flux. We send down a serialized version of a large object. In our case, we were doing flux.serialize() within createElement. The serialization method could take ~20ms. With 4 templates, that would be an extra 80ms to your renderToString() method!

Old code:

function createElement(Component, props) {
    props = _.extend(props, {
        flux: flux,
        path: path,
        serializedFlux: flux.serialize();
    });
    return <Component {...props} />;
}
var start = Date.now();
markup = renderToString(<RoutingContext {...renderProps} createElement={createElement} />);
console.log(Date.now() - start);

Easily optimized to this:

var serializedFlux = flux.serialize(); // serialize one time only!

function createElement(Component, props) {
    props = _.extend(props, {
        flux: flux,
        path: path,
        serializedFlux: serializedFlux
    });
    return <Component {...props} />;
}
var start = Date.now();
markup = renderToString(<RoutingContext {...renderProps} createElement={createElement} />);
console.log(Date.now() - start);

In my case this helped reduce the renderToString() time from ~120ms to ~30ms. (You still need to add the 1x serialize()'s ~20ms to the total, which happens before the renderToString()) It was a nice quick improvement. -- It's important to remember to always do things correctly, even if you don't know the immediate impact!