IMHO, there's a huge difference between calling a C library directly and shelling out to an external process.

mini_magick uses Subexec which in turn causes a fork and exec and might, depending on how ruby's backtick operator work, also start a shell which in turn does another exec for the specific ImageMagick binary.

The process startup time alone is a significant overhead and so is the additional memory usage.

In the old days, when I was maintaining a mail server for a free web mail service, I remember the lengths I went through to prevent my MTA from forking for mail delivery due to the huge cost of launching another process.