Python builds on arm64 don't seem to have pre-installed python

Here’s a build with Arm64 xenial:
This build took 2 min 1 sec

Here’s a build with AMD64 xenial:
This build took 33 seconds.

Basically everything between these In both cases, the primary time sink is bootup time; the tests themselves take under 2 seconds. The reason I was trying to use arm64 is because it’s supposed to have a <10 second bootup time according to the build environment overview which would be a substantial improvement for me.

However, it seems that the image doesn’t come with python pre-installed and has to download and install it, which takes about 20 seconds. Installing dependencies takes about 40 seconds, which is somewhat expected since this build wasn’t cached. But then there’s also about a minute of time in the overall 2 minute build that doesn’t seem to be accounted for in the build log.

Is there something that I’m doing wrong? Is it possible that python could start being pre-installed on the base xenial arm64 images? Would it maybe still be slower even if that were the case?