[Update 2015-08-25: I reran some of the tests using two different settings in VirtualBox. First, I explicitly set KVM as the paravirtualization mode (it was saved as 'Legacy' by default, due to a bug in VirtualBox 5.0.0), which showed impressive performance improvements, making VirtualBox perform 1.5-2x faster, and bringing some benchmarks to a dead heat with VMware Fusion. I also set the virtual network card to use 'virtio' instead of emulating an Intel PRO/1000 MT card, but this made little difference in raw network throughput or any other benchmarks.]
My Mac spends the majority of the day running at between one and a dozen VMs. I do all my development (besides iOS or Mac dev) running code inside VMs, and for many years I used VirtualBox, a free virtualization tool, along with Vagrant and Ansible, to build and manage all these VMs.
Since I use build and rebuild dozens of VMs per day, and maintain a popular Vagrant configuration for Drupal development (Drupal VM), as well as dozens of other VMs (like Ansible Vagrant Examples), I am highly motivated to find the fastest and most reliable virtualization software for local development. I switched from VirtualBox to VMware Fusion (which requires a for-pay plugin) a year ago, as a few benchmarks I ran at the time showed VMware was 10-30% faster.