is that possible to reduce build times under 40 minutes for ios build using mechine medium ? or need to upgrade ? currently build time average is 45 minutes
It would depend on what the bottleneck is for your build. Upgrading to a larger system would provide you with more cores and local memory, but if you are limited by the process being single-threaded, large amounts of disk I/O or external network traffic you would not see any major benefit.
As the larger system needs an annual contract it would be worth looking at what is causing the build to take so long first. Once you know what is taking the time it may also be worth you also looking at the Self-hosted runner option if you have in-house macOS based systems that perform better.