[ubuntu] FDE + bad HDD = instable system?

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http://ubuntuforums.org – I installed Ubuntu using the alternate installation disk and opted for the option of full disk encryption. However the OS shows unstable behavior. After a few minutes of using it, it reacts very slow. The computer sometimes (temporarily) freezes or even crashes when an application (like Firefox) is opened. I do use a pretty old (second hand) hard disk. However it worked fine when I installed Ubuntu without encryption on it. My question: is it possible that Ubuntu with full disk encryption is more vulnerable for bad hardware than a regular unencrypted OS? . (Hardware)