Well, that post covers 2 months, and doesn't have hundreds of thousands of users expressing concerns. I'm not an Android expert as it happens, but I'd guess that the people on that thread have real issues that the Android team are working on. I wouldn't suggest you worry about 5.x... just get started. When you have a major problem with performance, submit a bug report with the project, and full details about the devices you are testing on, then contribute to the forum thread.
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