I agree on the performance loss. Just for kicks, I ran the same commands on some real aarch64 (32 cores, 3.0GHz, ARMv8.? - can't remember and already logged off the machine, but I can double check tomorrow). Without further context, numbers:

  someuser@some-aarch64-machine:~$ docker run arm64v8/ubuntu bash -c 'dd if=/dev/urandom bs=4k count=10k | gzip > /dev/null'
  10240+0 records in
  10240+0 records out
  41943040 bytes (42 MB, 40 MiB) copied, 2.18298 s, 19.2 MB/s
  someuser@some-aarch64-machine:~$ docker run amd64/ubuntu bash -c 'dd if=/dev/urandom bs=4k count=10k | gzip > /dev/null'
  warning: TCG doesn't support requested feature: CPUID.01H:ECX.vmx [bit 5]
  warning: TCG doesn't support requested feature: CPUID.01H:ECX.vmx [bit 5]
  warning: TCG doesn't support requested feature: CPUID.01H:ECX.vmx [bit 5]
  10240+0 records in
  10240+0 records out
  41943040 bytes (42 MB, 40 MiB) copied, 6.72324 s, 6.2 MB/s