Max redo speed on replica side in Availability Group I heard about maximum 50 Mb/sec for receive and 50Mb/sec for redo. 
But really, I have only 20Mb/sec redo speed. This is speed from Perfmon, so I know about the following issue:


AlwaysOn Availability Groups log_send_rate


As I know redo operation depends on CPU and for SQL Server 2012 it's run use only one core of CPU. I think it’s awful for situation when I can generate a lot of logs. For ex. when reorganize a lot of indexes in big tables.

Do you know about any KB or advise for improving performance for this case?
"Heard about"? From where? As far as I know this is dependent on local factors such as raw disk subsystem capability, controller capacity, concurrency, type of data being logged, etc. etc. 

Redoing large index maintenance operations on AGs is definitely a known issue. This post certainly admits as much and suggests ways to mitigate the issues:


Recommendations for Index Maintenance with AlwaysOn Availability Groups


But I don't think you can expect any predictable x MB/sec never mind a guarantee, you will have to observe what your system is capable of, and troubleshoot issues when it falls below your threshold range.