A user over on the AXUG Community recently posted a question about poor performance on the standard AX Inventory reports, specifically Physical Inventory by Dimension. It turns out the fix ended up being to clear the records in two tables in the database that are used only for building this report. Normally the records are created, the report run, and then the records are deleted. However if the process is cancelled the records can be left behind. Multiple users reported the same issue.
If you are having this issue or even minor performance issues with these reports, look into deleting all of the records in the InventSumDateTable and InventSumDateTrans tables via SQL.
If you haven't joined the AXUG Community forum yet go check it out. It's a great free resource for users.