The fork+exec is efficient. The blog post compares things without units. Forks (principally page table copies w/copy-on-write in effect) are measured in microseconds and the exec is your standard binary startup time. While you don't want to put a synchronous fork/exec in the way of 5,000 reqs/sec, it will be a trivial part of your asynchronous imagemagick processing.

At scale, you might care about the imagemagick startup latency, but not the forking.