Re: [High-Vt, Low-Vt] Mix library synthesis -> what approach is better?
If you don't have any timing violations (in high frequency designs if the WNS of violations are less than 3% of clock period it can be fixed in layout as skew comes in picture) in your design, you can do synthesis only with HVT.
If there are any timing issues you can open either LVT during incremental synthesis or at the starting itself. But if you are opening LVT at the start of synthesis, then there has to be constrained imposed on the design related to power so that less number of LVT cells will be used.
In case of RTL compiler there are specific attributes which can be used for minimizing the usage of LVT cells even if LVT is opened at the start of synthesis.