I supposed it’s possible that the import is consuming so much resource (mostly SQL) that the report generator page can’t render, but I think it’s unlikely.
What symptoms are you seeing? Does the browser navigation bar just keep spinning? Are you getting an error message, a totally empty page, or a page with all the column headers that just doesn’t contain the data you expect (no rows)?
You could wait for the import to finish and try again, but I suspect that you may have built a combination of filters that that excludes all of the results. When building a new report, it’s sometimes useful to add or remove filters one at a time so it is easier to see if one filter excludes all the expected results.