gitlab-ci-multi-runner high CPU usage

It's not clear to me if this is the expected behavior, but running a single build with gitlab autoscale causes the gitlab-ci-multi-runner process to incur 6 minutes of CPU time. In addition to the CPU resources consumed, there is a minimum 6-minute wall-clock time for a no-op docker build job. This seems a lot more than what I would otherwise expect, and I seem to recall it being faster in earlier versions.

The time appears to be:

• ~4 CPU minutes during the line Pulling docker image dockerimage:latest...
• ~2 CPU minutes after the conclusion of the build and before uploading artifacts. This is the case even when no artifacts are uploaded.

During each of these periods, a single core on the multi-runner host is pegged as fast as the host machine can go.

$ gitlab-ci-multi-runner --version
Version:      1.6.1
Git revision: c52ad4f
Git branch:   1-6-stable
GO version:   go1.6.3
Built:        Mon, 03 Oct 2016 14:47:22 +0000
OS/Arch:      linux/amd64

$ uname -a
uname -a
Linux [hostname redacted] 3.16.0-4-amd64 #1 SMP Debian 3.16.36-1+deb8u1 (2016-09-03) x86_64 GNU/Linux

$ cat /etc/gitlab-runner/config.toml
concurrent=5

[[runners]]
  name = "[redacted]"
  url = "[redacted]"
  token = "[redacted]"
  executor = "docker+machine"
  tls-ca-file = "/etc/ssl/certs/ca-certificates.crt"

  limit = 5
  [runners.machine]
    IdleCount = 1
    IdleTime = 2700
    MaxBuilds = 100
    MachineName="ci-auto-%s"
    MachineDriver="amazonec2"
    MachineOptions = [
        "amazonec2-instance-type=t2.nano",
        "amazonec2-tags=ci-auto,yes",
        "amazonec2-vpc-id=[redacted]",
        "amazonec2-access-key=[redacted]",
        "amazonec2-secret-key=[redacted]",
        "amazonec2-ami=ami-4a12565d",
        "amazonec2-device-name=/dev/xvda",
        "amazonec2-ssh-user=admin"