Benchmark
LagAssist benchmark uses metrics from CpuBenchmark, memory information, along with some calculations to approximate the number of players your server will keep without lagging. It is still in development, but it still should work fine with most Linux & Windows OS-es.
(!) Your server needs to have access to connect to PassMark's Cpu Benchmark website. Also, please note that some hosts might not allow checking what CPU they are using (which is necessary for a quick benchmark lookup). If you have any questions, make sure to ask me.