Roel Schouwenberg is a senior anti-virus researcher in Kaspersky Lab s Global Research & Analysis Team. He recently started exploring the wonderful world of Solid State Disks (SSDs) Under certain circumstances, an SSD may actually perform better during an AV scan than when it s idle. The culprit seems to be related the C1E (HLT) state which is used in the vast majority of modern CPUs to save power. When the CPU is in this state the majority of chip sets will have significantly poorer I/O results for SSDs.
Source: https://threatpost.com/av-scans-slowing-down-your-machine-think-again-040510/73784/