LLVM has many advantages for JIT but it does not prioritize very low latency, choosing to optimize other properties instead. Consequently, LLVM tends to be more popular for systems targeted at analytical workloads where queries commonly have intrinsically high overhead latency. The latency isn't terrible but it is noticeable if you are running a low-latency "fast-twitch" workload.

There are other specialized JIT compilers that are essentially purpose-built to provide very low latency; for systems like MongoDB which are rarely used for serious analytical processing, you'd probably want to use one of these instead.