I/O tends to be the slowest operations your software can perform, and also the riskiest, because you’re dependent on so many different underlying components working correctly. Everything from the kernel syscall, to the device driver, the device itself, and potentially devices attached to device that’s attached to your computer. In short IO operations are a complete shit show of possible problems, that can all occur while your software is suspended in a syscall.

Memory allocation by comparison are extremely quick, and generally very reliable. Your system’s memory subsystem isn’t a smorgasbord of different memory drivers and controllers. It one memory system, taking to one memory controller, via an API that been standardised for decades, and where every implementation of that API is basically tested to the extreme every time a computer turns on. That’s assuming your language even bother asking the OS for memory on every allocation, which it probably doesn’t. Most language runtimes request large blocks of memory from the OS, then allocate out of those block on demand. So most “allocating functions” never result in syscall at all.