Are you solving CFD or Structural models? Since you posted in the Mechanical section I will assume you have a Structural model.
How many cores did you start with?
On a Structural model, the solve time has diminishing improvements with added cores and can actually increase if you use too many cores. The CFD models scale much more linearly with increasing core count than the Structural models.
How many cores are on one node? The communication speed between cores on the same node is an order of magnitude faster than the communication speed between nodes, so the best performance is when the job runs on a single node.
Don’t use all the cores on the node, that will increase the solve time. Use at least one less than the maximum number of cores if the core count is small. If the core count is high, a smaller number of cores might take less time.
Does the problem run in RAM (incore) or not? If the model doesn’t fit in RAM, it will be slowed down by that and adding cores might have little effect. If the model doesn’t fit in RAM, you should spend time reducing the size of the model until it will run in RAM.