There are too many fluctuations in bandwidth on the open Internet to test such a claim fairly. Instead, we decided to look around and figure out exactly what System Mechanic was doing in its attempt to fix network-speed issues. We compared the Windows Registry before and after installing and running System Mechanic 10 on the Lenovo ThinkPad Edge. The only Registry adjustment we found was a change in the Maximum Transmission Unit setting. Old-school PC gurus will recognize this particular tweak, as it's something of a classic--we ran an article about changing the MTU 13 years ago, for instance.
Unfortunately, no sophisticated Windows magic seems to be going on here. The optimal MTU for your PC depends on whether you're using an always-on Internet connection (such as standard DSL/cable), a PPPoE (PPP over Ethernet) broadband connection (if you have DSL/cable service that requires you to log in at every boot-up, you're probably using PPPoE), or a 56-kbps dial-up connection. System Mechanic simply asks you which type of connection you have during the setup process, and adjusts the MTU accordingly (1500 for a standard always-on connection, 1492 for PPPoE, or 576 for dial-up). You don't need System Mechanic to do this task, though: The oldie-but-goodie Dr. TCP will help you change your MTU for free.
Cleanup utilities are a compelling sell to Windows users. We all want to believe that our PC is still the same snappy spring chicken it was when we bought it, and that it just needs the light touch of a cleanup tool to start sprinting again.
The reality is a bit different: You might feel better after running a utility--but judging from our testing, your PC's overall performance is unlikely to change much. Instead of investing in a cleanup utility, uninstall old programs for a short-term speed boost, and save your cash for a hardware upgrade.