Tim Litton's blog post shares recent testing of Hyper-V and how disk workloads perform when using Fixed or Dynamic VHDs.

His graph shows perf across several scenarios (with Dynamic VHD being the baseline).

Hyper-V VHD Performance - Dynamic vs. Fixed

Fixed VHD always performs better than a Dynamic VHD in most scenarios by roughly 10% to 15% with the exception of 4k writes, where Fixed VHD performs significantly better.