You don't think the tests are cheap and fast to allow waiting lines to move faster and to save money?
Everything would move much slower if you had to wait 5 minutes in an 'expensive and slow' device.
I agree with you entirely - that's the correct kind of test to employ. My point is that getting a positive on such a test doesn't give you grounds to believe someone has handled explosives. Out of every 1000 positives provided by such a test, approximately 1000 are false positives, give or take 1.
The tests they use are intentionally cheap and fast. Accuracy is not the primary criteria.
For white people you get an extra 30 second pat down. For brown people you get detained and interrogated all day.
Like drug dogs, false positives may be considered a bonus by the security apparatus because it allows the apparatus to work against the discriminated-against class and blame it on something else, as you have just done.