Performance differences - measures on SSAS vs measures on Powerbi

For specifics on measure performance between different versions of Analysis Services (including the one used within Power BI) you should probably try the AS forums here. What I can tell you is there is likely a big difference between 2012 and Power BI AS. Depending on which version of SSAS 2012 you have the compat level supported by that server is either 1100 or 1103. Power BI is closely aligned with Azure Analysis Services/SQL AS 2017 which has a compat level of 1400. Even with the compat levels aside there have been a number of engine and DAX performance improvements over that last few releases of SQL AS. You can check this link which is a summary of all the "What's new" posts that the AS team has published on there blog, you can take a look at all of the enhancements since 2012. Hope this helps, thanks!