Question is: Can we actually review them?
I'm edging towards no, with the following arguments:
- Those type of performance issues are non deterministic, and are not always to be encountered even on the same PC. Some factors are simply CPU load, but also browser memory usage.
- The performance will vary from PC to PC, I for example barely ever experience the issues described in the questions, so I couldn't possibly try to fix those issues without investing an unreasonable amount of effort by obtaining a lower end PC.
Note that these performance questions are very different from say, sorting a list, as there you can make algorithmic improvements (do not need to be measurable) and other measurable improvements, as there the programmer controls mostly what the code does.
An example of such question which I would not find appropriate to review is:
CSS3 transitions performance enhancement
I am unsure though for exactly which reason it would be off topic, perhaps some kind of reason as SO has, along the lines of "Could not be reproduced".