Xcode build times very slow

I have two mac pro.

The A Mac Pro:

  Model Name: Mac Pro
  Model Identifier: MacPro6,1
  Processor Name: 6-Core Intel Xeon E5
  Processor Speed: 3.5 GHz
  number of Processors: 1
  Total Number of Cores: 6
  L2 Cache (per Core): 256 KB
  L3 Cache: 12 MB
  Memory: 16 GB
  Boot ROM Version: MP61.0116.B46
  SMC Version(system): 2.20f18

macOS Sierra 10.12.6

The B Mac Pro:

  Model Name: Mac Pro
  Model Identifier: MacPro6,1
  Processor Name: 6-Core Intel Xeon E5
  Processor Speed: 3.5 GHz
  number of Processors: 1
  Total Number of Core: 6
  L2 Cache (per Core): 256 KB
  L3 Cache: 12 MB
  Memory: 32 GB
  Boot ROM Version: MP61.0120.B00
  SMC Version(system): 2.20f18

macOS High Sierra 10.13.2

for the same project, A needs 7 minutes, B actually takes 15 minutes.

Configuration is higher, why is it slow? anyone can help me?