" Is there an easy way to delete most of these keyframes?"

There's the "Resample Key Frames" function that has an option to re-arrange the key frames and insert one at a user specified interval. It also has an option that allows Poser to analyze the amount of change over time in the animation and place key frames accordingly.

I've used the Resample Key Frames function to clean up walk cycles produced by the Walk Designer.

Standard warning applies: Back-up the animation before trying!

The resample function works well enough for walk cycles but could have unforeseen effects on more complex animation sequences.