Are you using R2020b? SDI performances are improved in R2021a. Also, you can configure the SDI to act as an oscilloscope, i.e. the data is only available during simulation (like the target scopes). Also, the host computer specs can significantly impact SDI performances. Tyically, the host-target Ethernet connection is not the bottleneck in this case.
Signals that are connected to scopes in the model are streamed to SDI. That may be the reason of seeing additional signals.
If any issue persists, I suggest to get in touch with technical support.