I haven't managed to duplicate the degree of slowdown that you report - I only get about 10%.
However I found an old Email which also reported slow raid5 in 2.6 and it noted that when the array is degraded, it goes faster ....
I just tried it and got most of the speed back when the array is degraded!!! Would you be able to test that option?
2 tests : (kernel 2.6.10-rc2 13d r5 ext2)
Version 1.03 ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
storage.a2000.nu 2G 25238 99 116958 98 67987 73 25702 91 142010 70 608.2 3
------Sequential Create------ --------Random Create--------
-Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
16 1354 78 +++++ +++ +++++ +++ 1262 74 +++++ +++ 4895 95
storage.a2000.nu,2G,25238,99,116958,98,67987,73,25702,91,142010,70,608.2,3,16,1354,78,+++++,+++,+++++,+++,1262,74,+++++,+++,4895,95
]# mdadm --manage --fail /dev/md0 /dev/sdn1 mdadm: set /dev/sdn1 faulty in /dev/md0 ]# mdadm --manage --remove /dev/md0 /dev/sdn1 mdadm: hot removed /dev/sdn1
Version 1.03 ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
storage.a2000.nu 2G 25458 99 107295 83 59079 60 26079 93 145775 77 490.2 3
------Sequential Create------ --------Random Create--------
-Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
16 1423 83 +++++ +++ +++++ +++ 1453 82 +++++ +++ 4872 100
storage.a2000.nu,2G,25458,99,107295,83,59079,60,26079,93,145775,77,490.2,3,16,1423,83,+++++,+++,+++++,+++,1453,82,+++++,+++,4872,100
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