Because Bigtable is rather complicated, reasons for its poor performance are often hard to find. Here are some pointers that might help you if you are suffering from a badly performing Bigtable:

• The first place to look would be at the schema design. Check if you have sequential keys causing hot-spotting, or causing the reads and writes to be concentrated in some specific shards. Some obvious keys to avoid are numeric sequences like 1011, 1012, 1013 and so on.
• The next set of possible causes has to do with inappropriate workloads. Maybe your dataset is too small, less than 300 GB. That is not enough for Bigtable to really show its talents. Bigtable is best used when there is more than 1 TB of data and it can be used up to petabytes in size.
• Another possible problem has to do with the usage pattern. Your queries may be running in short bursts. Bigtable performs best when it has hours of observations to tune performance internally.
• There are also some of the usual suspects, for instance maybe your cluster is just too small.
• Another possibility is that your cluster has just been fired up or just been scaled up. In either one of these cases, it is going to take some time for Bigtable to understand the patterns and allocate the newly added resources optimally.
• It might also be a case of you using HDDs instead of SSDs.