I have exactly the same problem with text boxes in Powerpoint on a computer I have used for loads of Powerpoint presentations for the last 3 years so must be an MS Update issue.

I restarted my computer but the same thing still happens, the CPU ramps up to 50% if I even type one letter in a text box.

It was on an existing presentation so I tried making a new text box in the presentation but the same thing happened.

I started a new presentation and that works OK.

I do a lot of work in Powerpoint and have a lot of existing presentations that I update frequently so I don't want to scrap all of that and redo everything.