Guest has partitions which are not aligned for best performance. Until around 2008, most guests installed themselves non-optimally like this, and this applies even when installing a pre-2008 guest today.
The guest suffers a performance penalty, as much as 20%.
libguestfs now provides two tools which can diagnose and fix these guests: virt-alignment-scan and (the updated) virt-resize.
Customers can use virt-alignment-scan to identify which guests may suffer from poor performance because of unaligned partitions. Furthermore they can fix these guests by using virt-resize. Customers should read the respective manual pages carefully first.