Are you testing on a x64 laptop? It could possibly be CPU throttling? Maybe try run the benchmark written in a non-JVM language to see if you get the same beahviour?
It's a mini-pc. Could be throttling. There are benchmarks written in tons of languages here: https://github.com/PlummersSoftwareLLC/Primes I've so far only seen this behaviour with the JVM involved.