You might try installing the Speed Patch to speed up your N900's performance in certain areas. - http://talk.maemo.org/showthread.php?t=73315 .

Also it is true that the Hildon Application Manager is slow, you can always switch to Faster application manager, although updating the packages from the repo might take time if you have the Extra-testing and Extra-devel repo's enabled. Usually it's better to have Extra-devel disabled if you'r not planning to try out unstable applications.