We just shipped Arq version 1.3.3 with support for selecting the “region” for your S3 “bucket”.
If you’re located in the EU, you might get better performance if you select “EU” for the region when setting up Arq.
Get Arq version 1.3.3 here.
You can also change regions if you’ve already got Arq running. Just go to Preferences -> Advanced and click the “Change S3 Region…” button. Unfortunately S3 buckets can’t be moved from one region to another, so if you change regions, you’ll be using a new bucket, Arq and Arq Agent will restart, and you’ll need to reconfigure which folders you want Arq to back up.