I'm highly skeptical that it was just offering bitcoin that reduced donations. For example, if they're making an API call to the payment provider on each page

I'm highly skeptical that it was just offering bitcoin that reduced donations. For example, if they're making an API call to the payment provider on each page load to generate a new bitcoin payment button, the page load might be slower, and it might be that additional slowness which accounts for the change. The conclusion was not scientific.