2.6.38.6-26.rc1.fc15.i686 is a kernel. The 2.6.38.6-26.rc1.fc15.i686 has been tested via the Phoronix Test Suite in the configurations listed below.
4123412 - Tests on Fedora release 15, Intel Core 2 Duo E8200, S3 Chrome 400, Gigabyte P35-DS3L
222222222 - Tests on Fedora release 15, Intel Core 2 Duo E8200, S3 Chrome 400, Gigabyte P35-DS3L
66666 - Tests on Fedora release 15, Intel Core 2 Duo E8200, S3 Chrome 400, Gigabyte P35-DS3L
tomo-20594-19893-26219 - Tests on PTS Desktop Live 2010.1, Fedora release 15, Intel Core 2 Quad Q9300, ATI RV410, ASUS P5P43TD, Intel Celeron 2.00GHz, ATI Radeon 9100 IGP
anon-3449-32336-29083 - Tests on Fedora release 15, Intel Core 2 Duo E8200, S3 Chrome 500, Gigabyte P35-DS3L
Ubuntu 11.04: i686 vs. i686 PAE vs. x86_64: At the end of 2009 I published benchmarks comparing Ubuntu's 32-bit, 32-bit PAE, and 64-bit Linux kernels. Those tests were carried out to show the performance impact of using 32-bit with PAE (Physical Address Extension) support, which on the plus side allows up to 64GB of system memory to be addressable from 32-bit machines, but is still significantly slower than a 64-bit kernel and user-space.