I'm currently using macOS Sierra.
macOS Mojave is working less faster than macOS Sierra on my Mac Mini 2014. But Apple told us that new macOS Catalina will be working faster. So, should I update my mac to the newest macOS?
Mac mini 2018 or later
You have a five-year old Mac. Apple is not going to change your hardware performance as it is fixed. Although there is a suggestion that Catalina should be faster by removing 32-bit code libraries and applications — bear in mind that Apple is also adding significant new functionality to the operating system. As usual, that will require more demands on your hardware, not less, and Apple's marketing communication about faster likely applies to those with recent or new hardware.
Only you know what you perceive when running macOS, as to whether it is faster or slower. Don't upgrade to Catalina without a last Time Machine backup of Mojave. Do not reuse that Time Machine drive for new Mojave backups. You can then use that former Mojave Time Machine drive to retreat back to Mojave via a Recovery boot.
To determine what is causing your Mac to run slowly please run Etrecheck It is a diagnostic tool that's very useful to us in finding problems. Also it will give us further specs on your Mac. After it runs post the log file here. It will contain no personal information. Allowing full drive access will improve the quality of the report.
To post the log file click on share report and use the Page icon in your reply window.
Only you can decide whether to upgrade or not. If it were me I would figure out what is causing my computer to be slow before doing the upgrade.
which OS will work faster: macOS Catalina or macOS Sierra?
