I found this out yesterday.
If using OST with certain storage providers (im my infrastructure Quantum DXi) there can be a hard coded CPU thread limit for Optimised duplications.
On the DXI range this is 8. Any more than 8 Optimised duplications running at once will reduce the performance significantly, any more than 20 and the appliance dies.
It doesn’t appear there is a way (yet) of limiting active SLPs in Netbackup. You can do this with Vault with a bit of tinkering (8 Vault Profiles and sub jobs under them).
Hope you find this useful!