This feature has been deprecated after Percona Server 5.1.66-14.2.
InnoDB writes a checksum at the end of each data page in order to detect data files corruption. However computing this checksum requires CPU cycles and in some circumstances this extra overhead can become significant.
XtraDB can use a more CPU-efficient algorithm, based on 4-byte words, which can be beneficial for some workloads (for instance write-heavy workloads on servers that can perform lots of IO).
The original algorithm is checked after the new one, so you can have data pages with old checksums and data pages with new checksums. However in this case, you may experience slow reads from pages having old checksums. If you want to have the entire benefit of this change, you will need to recreate all your InnoDB tables, for instance by dumping and reloading all InnoDB tables.
Once enabled, turning it off will require table(s) to be dump/imported, since it will fail to start on data files created when innodb_fast_checksums was enabled. In this case ALTER TABLE won’t work due to its implementation.