vdh `get_framerate` is very slow when a video document has non-`file://` scheme

[keighrim]

Bug Description

Needs further investigation, but when processing a non-file:// VideoDocument (e.g., baapb:// ) converting between time units and extracting still images takes unbearably long time, most likely due to slow-down in getting the FPS information using cv2.

Reproduction steps

Expected behavior

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Log output

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Additional context

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