@swcheppes The DISM error means you chose the regular version of CommandPrompt or PowerShell. Be sure to pick the one that has the word "admin" in it.
The question about how a new save runs is meant to distinguish between issues with your user data, including your existing saves, and system issues. If a new save lags, it's a system issue. If a new save is fine, there's something about your older saves that's causing lag. There are ways to troubleshoot each, but they're totally different, which is why it's important to test a new save.