He's right.

Presumably, all of the processor cores on the web server are already devoted to handling user requests. If you process these items in parallel on the web server, the most likely outcome is that performance will not improve unless the web server is lightly loaded. You might even see a slight performance degradation under heavy load.

One way to solve the problem is to offload the parallel processing onto another computer.

He's right.

Presumably, all of the processor cores on the web server are already devoted to handling user requests. If you process your hot path in parallel on the web server, the most likely outcome is that performance will not improve unless the web server is lightly loaded. You might even see a slight performance degradation under heavy load.

One way to solve the problem is to offload the parallel processing onto another computer.