fork a child process for that specific task, you can even create a queu to limit the number of child process that can be running in a given moment.

Here you have a basic example of a worker that sends the original express req and res to a child that performs heavy sync. operations without blocking the main (master) thread, and once it has finished returns back to the master the outcome.

Worker (Fork Example) :

process.on('message', function(req,res) {
  /* > Your jsdiff logic goes here */
  //change this for your heavy synchronous :
  var input = req.params.input;
  var outcome = false;
  if(input=='testlongerstring'){outcome = true;}
  // Pass results back to parent process :
  process.send(req,res,outcome);
});

And from your Master :

var cp = require('child_process');
var child = cp.fork(__dirname+'/worker.js');

child.on('message', function(req,res,outcome) {
  // Receive results from child process
  console.log('received: ' + outcome);
  res.send(outcome); // end response with data
});

You can perfectly send some work to the child along with the req and res like this (from the Master): (imagine app = express)

app.get('/stringCheck/:input',function(req,res){
 child.send(req,res);
});