Innobackupex (Xtrabackup) 2.4.8 incremental backups take the same time as full backup
It's kind of expected assuming how incremental backups work.


Xtrabackup knows the last LSN (a point when a full copy was taken).
Xtrabackup reads all pages. Every page has an LSN in its header of the latest modification:


if a page is newer than the last LSN - save it.
if a page is older than the last LSN - skip it.



So, in the end of the day xtrabackup needs to read same amount of pages, whether it's a full or incremental copy.

But. There is an option in Percona Server can track modified pages. In that enabled, Xtrabackup doesn't have to scan whole tablespace and thus incremental backup time can be greatly reduced.