We noticed a regression with IOzone when performing 512MB and 1GB writes on Ubuntu 9.04, but the system performance was still very much improved overall. The only other area where there are performance regressions to cause concern is with the Intel integrated graphics. Intel's GEM implementation and other work could have been more stabilized until they released it, but they did not, which is causing some temporary pain for Linux desktop distributions. With Canonical likely enabling greedy migration heuristics in time for Ubuntu 9.04 and perhaps some other tweaks, this will be good news and should lead to better results. If you are just using your Intel netbook for surfing the Internet, e-mails, word processing, and other desktop applications, these regressions should go mostly unnoticed though. Compiz is also still plenty fast on Ubuntu 9.04 with the Inspiron Mini 9.
Where there were not graphics regressions, there were large performance boosts. Even before getting to the desktop benchmarking, the boot performance on Ubuntu 9.04 was tremendously better than on Ubuntu 8.04.2 LTS and Ubuntu 8.10. With Ubuntu 8.04.2 LTS it took 27 seconds to boot, but only 14 seconds on Ubuntu 9.04! When it came to the desktop, Ubuntu 8.10 and 9.04 were dramatically faster with the timed ImageMagick compilation, 7-Zip compression, LZMA compression, SciMark, C-Ray, RAMspeed, and some of the graphics tests. Many of these performance boosts will be very evident to the end-user.
Besides running faster, the newer Ubuntu releases have better hardware support, various package updates, many improvements to GNOME, and all sorts of other features. Ubuntu 8.04 is nice for its Long-Term Support with Canonical continuing to push out security updates for years to come, but a much better experience can be had on Ubuntu 9.04 with its faster performance, updated packages, and newer features. Ubuntu 8.04.2 is faster than Microsoft Windows, but the newer releases are even faster! Ubuntu 9.10 should be especially interesting on netbooks with the Intel Linux graphics stack hopefully being settled by then, the integration of Plymouth and kernel mode-setting, and its use of hopefully X.Org 7.5 and the Linux 2.6.31 kernel.