SDL does at least partially use DX, when used in Win32, FYI.
And just because something uses SDL, doesn't mean it's going to immediately compile on another platform. There are many other aspects of a codebase which could be in a non-portable state..
And yes, SDL isn't the speediest of choices, but that's often a sacrifice you make for ease of implementation, and portability.
Anyhoo, any decent machine should run it at full speed, if it's ass-slow for you, you should try using a newer SDL library, or getting rid of your crapbox
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