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If your Droid Maxx has (presumably!) the Android OS on it, then if that OS version is not sufficiently updated, then it may contain a significant 'security hole' that allows almost anyone to gain access and control over your phone.
To test as to that security hole, download and use this app:
to tell whether there is such a security hole in your phone's Android operating system.
To avoid the problem in the first place, not just using the Stagefright detector, but also completely updating your Android OS ab initio when you first start using it, is a good idea. (Some older versions of Android OS cannot be patched, by merely updating that version... but a higher version, if it can run on the older Android OS -compatible phone, can cure that problem.)
If the Stagefright exploit is a vulnerability in your phone as it is, someone - some eager beaver of a nice little felon - may have exploited it, to access and control your phone. Running very slow is often a symptom of a computer device's operating system being hacked, or infected with virus(es), esp. if it was doing fine before.
I'm sorry to say this. Android's discovered vulnerability with regard to the Stagefright exploit or security hole, is some of the worst news in years in the computing device security world. (And quite a cachet to the Apple iXXX device enthusiasts, though Apple devices have their weaknesses too, sometimes.)
It could possibly be a hardware problem. Have you filled up all available memory on the phone? that's not likely to be a cause, but might be a symptom. Did the slowness set in fairly suddenly, or just gradually over weeks or months, getting worse?
I did not know there were a choice of three detectors. What are they called?