Using a kernel which will use only a single physical and logical (hyper-threading) CPU core implicitly protects against attacks which involve multiple CPU cores, i.e. certain types of local side-channel attacks, race conditions etc. But it also drastically reduces the performance on multi-core systems which makes it more susceptible against attacks where the defense could profit from more performance or parallel execution.
Thus, if you need a "safer" operating system you should evaluate in detail how safe it really needs to be and what kind of attacks are actually relevant. Everything is a trade-off and if you optimize to protect against some kind of attack which is not relevant for your use case you might inadvertently make you more vulnerable for attacks which are actually relevant.
Specifically, if your use case does not involve running potentially untrusted code on the system (like Javascript from somewhere on the internet running inside the browser), then protecting against local side-channel attacks is probably not relevant, especially if the protection dramatically reduces the performance of the system.