
There could be many possibilities, but since you mentioned this is related to Inventory Valuation, these reports are generally heavy and can take time even in a normal environment.
You can first check the Performance Profiler/Performance Matrix with only one user logged in. It would also be a good idea to review the tenant health.
My recommendation would be to assign this report to a Job Queue and run it during off-business hours. Once the report is generated, it can then be sent or triggered for the corresponding users.
Normally, for Inventory Valuation, AR, and AP reports, this is considered the best approach.