Just posting my final resolution in case it helps others with similar performance issue. Fluxbox allows toggling 'opaque window moving' (Openbox only supports this for window resizing, not moving). So Fluxbox is my new favourite for this old hardware, providing similar performance to JWM with the benefit of most Openbox features.
Although the same performance issue was evident in the default Fluxbox and Openbox configurations, performance after toggling opaque window moving in Fluxbox is very crisp. Web search indicates this feature was suggested to Openbox numerous times over the years, never implemented
Another issue that initially kept me away from Fluxbox was not being able to automagically open new windows under the mouse cursor, although i've since discovered it is supported by changing .fluxbox/init to:
session.screen0.windowPlacement: UnderMousePlacement
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