If you use ::getEngine() instead of ::newDifferentialMarkupEngine(), you'll reuse the existing default engine, which is likely appropriate here. It won't include differential.custom-remarkup-rules or differential.custom-remarkup-block-rules, but those are slated for deprecation soon.

This is better than what we were doing before, but it's still basically an uncached, unbatched process.

You can cache and batch processing with PhabricatorMarkupOneOff. For each field, do:

$markup_object = id(new PhabricatorMarkupOneOff())
  ->setContent($userdata);
$engine->addObject($markup_object, 'default');

After adding all of the text, do;

$engine->process();

Then, retrieve the markup results with:

$output = $engine->getOutput($markup_object, 'default');

So, overall, it would look something like this:

$markup_objects = array();
foreach ($udata as $key => $test) {
  $obj = new oneoff ...
  $engine->add($obj, ...
  $markup_objects[$key] = $obj;
}
$engine->process();

foreach ($udata as $key => $test) {
  $fancy_userdata = $engine->getOutput($markup_objects[$key], ...
}