Diagnosing poor build performance

I have a 28-core Mac Pro which takes twice as long to build my project as a 16" macbook pro. I want to understand why.

I'm building this project: https://github.com/gnachman/iTerm2

The build configuration is Debug. The optimization level is set to None. There is no Swift: just C, Objective C, and interface builder XIBs. I'm using Xcode 11.3 on both machines on macOS 10.15.1.

Build time summary for Mac Pro:

CompileC (728 tasks) | 1673.157 seconds
CompileXIB (58 tasks) | 649.565 seconds
TiffUtil (121 tasks) | 36.980 seconds 
ProcessPCH (5 tasks) | 16.840 seconds 
CopyPNGFile (26 tasks) | 12.200 seconds 
StripNIB (1 task) | 9.133 seconds 
CompileMetalFile (21 tasks) | 6.290 seconds 
CompileAssetCatalog (1 task) | 1.688 seconds 
Ld (2 tasks) | 1.528 seconds 
Libtool (3 tasks) | 1.098 seconds 
CodeSign (7 tasks) | 0.768 seconds 
PhaseScriptExecution (1 task) | 0.248 seconds 
MetalLink (2 tasks) | 0.222 seconds 
DataModelCompile (1 task) | 0.171 seconds 
ProcessPCH++ (2 tasks) | 0.053 seconds 
Touch (1 task) | 0.003 seconds

Build time summary for Macbook Pro:

CompileC (728 tasks) | 290.118 seconds 
CompileXIB (58 tasks) | 12.626 seconds 
ProcessPCH (5 tasks) | 9.958 seconds 
TiffUtil (121 tasks) | 5.955 seconds 
CompileMetalFile (21 tasks) | 5.135 seconds 
CompileAssetCatalog (1 task) | 1.283 seconds 
Ld (2 tasks) | 1.143 seconds 
CopyPNGFile (26 tasks) | 1.102 seconds 
CodeSign (7 tasks) | 0.734 seconds 
Libtool (3 tasks) | 0.332 seconds 
PhaseScriptExecution (1 task) | 0.197 seconds 
DataModelCompile (1 task) | 0.196 seconds 
MetalLink (2 tasks) | 0.083 seconds 
ProcessPCH++ (2 tasks) | 0.046 seconds 
StripNIB (1 task) | 0.012 seconds 
Touch (1 task) | 0.001 seconds

Observations:

* Clock time is 52.2 seconds for the Mac Pro vs 27.6 seconds on the Macbook Pro.

* CPU utilization on the Mac Pro is abysmal. It stays around 30%. The macbook pro stays at 100% while building.

* No throttling, per Intel Power Gadget. The average clock rate stays at 3.4ghz while building.

* CPU temperature never exceeds 60 degrees C.

* I get the same results when source and build folders are in a ramdisk on the Mac Pro.

* I/O doesn't seem to be a problem, as shown by iostat: https://pastebin.com/avjaQ3ZE

* I have tried reducing parallelism by setting the IDEBuildOperationMaxNumberOfConcurrentCompileTasks user default, but it doesn't have any effect.

* I made a script that injected timestamps into the command-line xcodebuild, which can be seen here: https://pastebin.com/yWdN27S8

My best theory is that so enough of the work is not parallelizable that the lower base clock rate is the problem, but I'm surprised how big the effect is considering there are over 600 translation units.

I would like to learn more about how to diagnose such an issue, since I'm pretty much out of ideas of what to try next.