Like antivirus apps, there’s no reason for users to download or trust “maintenance” or “utility” apps. That includes apps that claim to speed up your iPhone, clear out memory, increase battery performance or defragment your iPhone’s drive is suspect. Think twice before downloading them.
By default, iPhones do a great job at managing memory and other performance needs without any third-party help. More than that, because they’re sandboxed, third-party apps don’t have access to the system features they’d need to actually speed up performance. Any “boost” in ability is likely just a placebo.