I've tried several things that have been suggested - turning off spell check as you type did nothing. Seems the problem is related to opening existing files - don't have this slowness issue when I open PPT and start a new presentation.

sometimes I found certain backgrounds made PowerPoint run slowly I don't know why but I changed the background back to blank and it worked again

I am having the same problem. As I moved to Powerpoint 2010 the typing went where slow, letters not showing before I stopped typing. It does indeed seem to be related to the background and more particular with a gradient background. As soon as I change the background to a solid colour everything was fine. Making a new presentation based on the template with gradient background resulted in the same sluggishness when typing. 

I have a same problem with powerpoint... the problem appears only when you edit an exisitng file...new files work smooth 

Hi all,

Based on my research, this issue may caused by the incorrect installed printer driver. Even though you never tried to print the presentations.

To solve this issue, try to uninstall and reinstall the latest printer driver.

Let me know the result, need any further help, feel free to post back.

 The default printer was not available and Powerpoint started responding extremely slow while typing. Switched to another printer and everything started going fast again.

I just was able to resolve this type of issue for my case. I was able to keep almost my entire background.

Issue:
Powerpoint 2010 cannot display typed characters at normal typing speed. (up to 1 second delay)
Slide transitions take about 0.5 to 1 second.

Cause: Slide Master contained a small graphic that caused slow response.

Resolution:View Menu -> Slide master
From slide 1, delete graphic.
View Menu -> Normal

System: Windows 7 64-bit (issue did not occur with Powerpoint 2010 on Windows XP, same file)

You may have to try deleting a few different items before finding the culprit.