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There's indeed some rate limiting these days.
In Re: Is BGG a lot slower these days? Or is it just me?, Aldie wrote:
we do think the increased number of API users is having a detrimental effect on the site. Many API developers show no regard for usage, and we're going to be remedying that with API keys and rate limits.
and a little later:
As long as you aren't exceeding 2 requests per second you should be ok.