steps to reproduce

1. run the attached zul, composer, and Excel file with 3.8.3.FL.20160307-Eval
2. click the "rebuild" button

result

• firefox show unresponsive warning and never complete after clicking "continue"
• in the user's firefox, it completes the js execution but takes much longer (521s) than 3.8.2 (12s).
  
  

expected result

around 12s like the previous version.

debug info

• 3.8.2.RC also has this issue.
• the time spent at the server has no big difference between 3.8.2 (20.6s) and 3.8.3fl (21s)
• 383fl's response size (991k) is much larger than 382 (13K)
• according to chrome profile, the client takes much time at adding and removing merged regions.