In my case 'the fix' was to put whole-core constraint on most utilized ldoms (databases) and keep the VCPUs count inside core boundary.
For instance, t5-2 sparc has 256 available VCPU (threads), which translates into 32 cores or 2 sockets 16 core each. For best performance one should give VCPU resources multiples of 8.
Be sure to reboot the hypervisor after such major changes.
Regarding HPVM (now vpars and Integrity VM) i would recommend using VPAR since they are configured only is such manner(dedicated cores for virtual machines and hypervisor).
Integrity VM can suffer from such 'misconfiguration' as well since it (can) share cores.
Nice blog about it, a bit old but good.
https://blogs.oracle.com/jsavit/entr...ore_allocation