I know this is a few months late, but my coworkers and I have been experiencing this issue as well. I contacted support the other day when the slowness evolved into issues fully deleting and importing datasets despite saying the requests were complete. Support gave me a solution which allowed me to solve this problem the next time it arose. Add a new embedded data variable in the Survey Flow. This forces the dataset to re-index. This seems to resolve any lingering requests which seem to be complete but actually aren't. You can remove the embedded data afterward. I hope this solution helps others!
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