Hello,
Only increasing the cores alone won't help. If the core count is increased too much, it will take significant time to pass messages across the cores. and it may end up taking more time. Also, if the mesh is same then at one point you won't see a huge difference in the time even if you use multiple nodes. Apart from these, there can be another reason too for the slow down.
For more information, please refer to Fluent User's Guide - Part III: Solution Mode (ansys.com)
If you are not able to access the above link, please follow this forum discussion - How to access the ANSYS Online Help
Hope this helps you. Thank you!
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