In addition to some iPhone users, several iPad users are reporting slowness after the update to iOS 4.2.1. Note that these performance problems may resolve themselves after a few minutes or hours as Spotlight finishes indexing the device. Failing that, try these fixes:
Perform a “hard reset.” Hold down the sleep/wake and home buttons simultaneously for roughly 15-20 seconds, until the screen powers off then an Apple logo appears, which signifies a reboot. Some users have (oddly enough) reported that performing two hard resets resolves the slowness issue.
Restore, but not from backup. It appears that bad holdover data from iPad backups can cause performance problems. Restoring as a new phone will delete contacts and other data, but may resolve this issue.
To do so, connect your iPhone or iPod touch to your computer, click “Restore” in iTunes, then choose setup as new device.
Free up space. Make sure that your iPad has at least 10% of its available memory free. OS X-based systems, such as iOS 4.2, may require some free space to operate properly.
Close open Safari windows. One iOS application that consumes memory in the background is Safari. Close all unused windows in the application by pressing the page switch button in the lower right portion of the screen then clicking the X at the top left of each page. See this page for screenshots.
Turn off Spotlight. Navigate to Settings -> General -> Home Button -> Spotlight Search and turn off all the options by tapping each one. Next, perform a hard reset by holding down the sleep/wake and home buttons simultaneously for roughly 15-20 seconds, until the screen powers off then an Apple logo appears, which signifies a reboot.
You can experiment with turning off only some options, but most users only report a meaningful speed boost if all options are turned off.
Downgrade to iOS 3.2.2. Connect your iPad to your computer, then select it in iTunes. Hold down the option key (on a Mac) or shift Key (in Windows) and click “Restore,” then select the old iPad 3.2.2 file (.ipsw), which should be named something similar to “iPad1,1_3.2.2_7B500_Restore.ipsw” from the following locations:
Mac: ~/Library/iTunes/iPad Software Updates
Windows: C:\Documents and Settings\<USER NAME>\Application Data\Apple Computer\iTunes\iPad Software Updates
If the file is not in one of these locations, it may be in your Trash (Mac) or Recycle Bin (Windows). Move it out, then perform the same process in iTunes, but select the .ipsw file from the location you moved it to.