What's fast on Z platforms is typically IO rather than raw CPU - the platform can push a lot of parallell data. This is typically the bottleneck when compiling.

The cores are in my experience moderately fast at most. Note that there are a lot of licencing options and I think some are speed-capped - but I don't think that applies to IFL - a standard CPU licence-restricted to only run linux.

Probably because that's just the infrastructure they have.

i686 builds even faster