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It seems that Poloniex takes up to 2-3 minutes (with quite a lot of variance) to compile the OHLC data (API method returnChartData) of a period that just ended. I've experimented with 30 minutes periods, so 2-3 minutes is up to 10% of a period, which is a lot!
Could it be that I'm doing something wrong? I'm using this python-poloniex wrapper, and I'm after the OHLC prices of a few BTC pairs.