diff --git a/lib/fs/reader_at.go b/lib/fs/reader_at.go
index 6a39865fb..ef30eb019 100644
--- a/lib/fs/reader_at.go
+++ b/lib/fs/reader_at.go
@@ -14,7 +14,8 @@ import (
 )
 
 var disableMmap = flag.Bool("fs.disableMmap", is32BitPtr, "Whether to use pread() instead of mmap() for reading data files. "+
-	"By default mmap() is used for 64-bit arches and pread() is used for 32-bit arches, since they cannot data files bigger than 2^32 bytes in memory")
+	"By default mmap() is used for 64-bit arches and pread() is used for 32-bit arches, since they cannot read data files bigger than 2^32 bytes in memory. "+
+	"mmap() is usually faster for reading small data chunks than pread()")
 
 const is32BitPtr = (^uintptr(0) >> 32) == 0