Comment by bheadmaster

In order to have direct function calls, you need to load the plugin's code in your own memory. Which means you expose your own memory to the plugin. Any malicious/buggy plugin could wreak havoc on your program - even managed code doesn't solve that problem in general.

IPC is the natural solution to that - run a process in its own address space and communicate with it via I/O. TCP is not most optimal, but it is uniquous. Same thing with JSON.

In LSP, most of the processing happens in the LSP server anyway - communication overhead between client and server is negligible in comparison to it.