To be nitpicky -- The AS/400 is not a mainframe, it's what IBM calls a "midrange system" positioned between commodity x86 servers and mainframes. Today, the AS/400 OS (OS/400, or as it's known now, IBM i) runs on IBM's POWER servers. For a typical IBM i system you're talking a couple 10s of thousands of dollars, for a typical mainframe you're in the hundreds of thousands.
Also regarding the power of mainframes: in the case of AS/400, it's definitely not what I would call fast. Everything runs on what is essentially a virtualization layer... the AS/400 instruction set (MI, or "machine interface") doesn't exist in any hardware, it's just run on top of a translation layer on top of (these days) POWER. It's a very weird system from top to bottom.
As for actual mainframes, traditionally the power came not from raw CPU power, but from an architecture that efficiently handles parallel I/O. Definitely powerful for batch processing of large amounts of data, but not necessarily optimized for something like games.
Also regarding the power of mainframes: in the case of AS/400, it's definitely not what I would call fast. Everything runs on what is essentially a virtualization layer... the AS/400 instruction set (MI, or "machine interface") doesn't exist in any hardware, it's just run on top of a translation layer on top of (these days) POWER. It's a very weird system from top to bottom.
As for actual mainframes, traditionally the power came not from raw CPU power, but from an architecture that efficiently handles parallel I/O. Definitely powerful for batch processing of large amounts of data, but not necessarily optimized for something like games.