Currently, Vivado Synthesis treats the instantiated cores as black boxes.
At present, Vivado Synthesis does not synthesize or optimize encrypted or non-encrypted synthesized netlists.
Hence, XDC constraints or synthesis attributes will not have effect for synthesis on an imported core netlist.
Also, Vivado Synthesis currently does not read the core netlist and modify the instantiated components by default.
The optimization of instantiated netlists will be supported in a future release.
However, Vivado Synthesis does synthesize secure IP RTL, and constraints do affect synthesis results.