Firmware 5.9.6 & Arabic e-books

Amazon's 5.9.6 firmware added support for Arabic e-books. Below are my impressions after testing the feature:

1. E-books purchased from Amazon worked well. I sent two sample e-books from the Kindle store and the text aligned right to left. By default, two fonts were supported and even side-loaded fonts worked too. It is also possible to look-up definitions and use an Arabic keyboard to write annotations. Font bolding, however, is not supported in Arabic e-books. 
2. I wirelessly sent two MOBI e-books and had significant problems. The major issue was the time it took to open an e-book - in both documents tested the Kindle completely froze. Further, even basic functions, e.g., turning pages, stalled the device. I further tested a side-loaded e-book and had the same issues. 

I am not sure if this is a problem with the Kindle firmware, as I didn’t have these issues when I opened the same e-books using an Amazon Fire tablet. I think there is something wrong with firmware 5.9.6 and Amazon could be working on a further update to resolve these problems.