I've just started a new project and am having a major problem with playback
speed. Clips that are 5 seconds long take several minutes to play back, making
editing impossible. This problem occurs with all new projects, but not with
old ones, which suggests that there is a playback speed option somewhere that
has gotten set to a bad value. I can't find such a setting anywhere. I've
googled this problem and found several requests asking how to slow down playback
speed to help with specific types of editing issues, but no answers. Could
somebody please advise me on how to restore my playback speed to normal real
time? I'm running into a hard deadline and won't be able to finish the project
without a solution.

This problem is not a proxy issue, I'm using proxies with
both old (working) and new (nonworking) projects.
Projects consist of a series of still images

kdenlive version is 20.04.2
OS is Fedora 31 running on an HP Pavilion A6209n

Additionally, I see that on the new projects with the speed problem, if I put
the timeline cursor in a new location, it takes about 5 seconds for the image
in the project monitor to update, a bit less if I select a part of the timeline
that is empty, and that on older projects without the speed problem, the
project monitor updates immediately.

I have edited this post to remove previous incorrect diagnoses of the playback
problem. I apologize if they caused any confusion. The correct solution follows.
The first time you add a clip to the timeline, kdenlive pops up a dialog which
says switch to clip profile (resolution speed fps). f you choose switch
rather than cancel, your project file will be corrupted, and playback will be
so slow as to be unusable.