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The feature is documented as: (from http://www.cisco.com/en/US/docs/ios/12_3t/12_3t8/feature/guide/gt_vfrag.html)
Restrictions for Virtual Fragmentation Reassembly
Performance Impact
VFR will cause a performance impact on the basis of functions such as packet copying, fragment validation, and fragment reorder. This performance impact will vary depending on the number of concurrent IP datagram that are being reassembled.
Automatically Enabling or Disabling VFR
VFR is designed to work with any feature that requires fragment reassembly (such as Cisco IOS Firewall and NAT). Currently, NAT enables and disables VFR internally; that is, when NAT is enabled on an interface, VFR is automatically enabled on that interface.
However, surprised this doesn't show a difference for CPU usage, especially as it's noticeable impact to performance, as you've described. Also surprising if you were NOT dealing with fragments, although the above does mention packet copying. Glad Patrick noted this.
