The fast-preview extension

The options below control the operation of the fast-preview extension that is run when you include "fast-preview.js" in the extensions array of your configuration. They are listed with their default values. To set any of these options, include a fast-preview section in your MathJax.Hub.Config() call. For example

MathJax.Hub.Config({
  "fast-preview": {
    Chunks: {EqnChunk: 10000, EqnChunkFactor: 1, EqnChunkDelay: 0},
    color: "inherit!important",
    updateTime: 30, updateDelay: 6,
    messageStyle: "none",
    disabled: false
  }
});

would ask for the PreviewHTML output to run as a preview (disabled: false), force inheritance of the surrounding text color, and set the updateTime and updateDelay to low values (30ms / 6ms) to speed up the preview pass. Furthermore, it configures the second pass to set the chunking (Chunks) to a reflow every 10,000 equations and disables the progress messages (messageStyle).

This extension provides a two-pass rendering mode. A first, fast-but-rough rendering is genereated as a preview, using the The PreviewHTML output processor, then a second pass using the configured output jax runs to update the preview output.

This two-pass mode will provide the reader with a quick, decent rendering to start reading immediately, while silently updating that rendering with the high quality layout later.

EqnChunk: 10000
EqnChunkFactor: 1
EqnChunkDelay: 0: These values control how “chunky” the second pass will be. For more information see The HTML-CSS output processor and The SVG output processor.

color: "inherit!important": This value allows you to choose a text color for the first passs.

updateTime: 30
updateDelay: 6: These values control how often the second pass will pause to allow user interaction (e.g., scrolling).

messageStyle: "none": This value controls the verbosity of the processing messages during the the second pass; see The Core Configuration Options for more information.

disabled:false: This value enables or disables the preview mode. In particular, it allows overriding a combined configuration file, cf. Combined Configurations. The user can independently enable or disable the fast preview via the MathJax Menu.