At Discourse our biggest constraint we are fighting now is memory, not performance (don't get me wrong, I wish MRI was faster). JRuby is usually more memory hungry than MRI and getting reasonable boot time is not easy on JRuby.
Additionally we don't even boot on JRuby today, I wish we did but it is hard to justify effort here.
RUBY_GC_HEAP_GROWTH_MAX_SLOTS is critical without it you get way too much bloat. The default is very problematic I discussed this with ko1 but not sure if we can get a better default in for 2.4.
Memory is indeed cheap but the problem is when you run Ruby web services you have every worker taking a minimum of ~400mb. This doesn't sound like much but then with micro-services and other APIs also using that much RAM per worker it adds up fast as all of these servers also need to take this much RAM per concurrent connection you would like to support.
For people starting out that is not so much a problem, but if you have moderate to large traffic you need a lot of machines just for the RAM. This is coupled with the fact that many Ruby libraries eschew streaming of files and connections and put things straight into memory despite Ruby having excellent block constructs that easily allow chunking/streaming. So apps also spike in memory and they need a large overhead as well.
Additionally we don't even boot on JRuby today, I wish we did but it is hard to justify effort here.