"Slow" and "fast" are relative terms. Neither of them will cause instructions to take a noticeable amount of time to execute, as long as they're not used excessively. So while you wouldn't want to be performing 500 long box traces each cycle, 50 short ones would be just fine. (I haven't stress tested that function so I don't know what the hard limits are, but basically use the "slow" functions sparingly and you'll be fine)

As for your question: c_scan() will only detect entities, not level geometry, so I wouldn't suggest you use that (unless your levels are made out of models). I would suggest you use a c_trace instruction in combination with the USE_BOX flag to perform a box trace upwards. That will accurately detect all surfaces above the "head" of your entity.