There's a wide range of ARM processors, but you can maybe sort them into three tiers: small AVR-like scale (e.g. Cortex-M), the smartphone speed (e.g. Cortex-A8

There's a wide range of ARM processors, but you can maybe sort them into three tiers: small AVR-like scale (e.g. Cortex-M), the smartphone speed (e.g. Cortex-A8/9 what most people come into contact with), and edging it's way into the market - the server targeted ARMs. The small, deeply embedded scale usually runs bare-metal or a small RTOS, while the next tier up often runs Linux. Now the small scale ARM is basically on the fast side (32bit proc) the very low end embedded cpus class. But it's often selected for everything but the most cost or power sensitive applications which tend to go for very small, power sipping 8bit cpus (e.g. MSP430s)