;DIronically, I actually agree with this. Hyper fast practice is what I think helps gain speed in the first place. It ramps up your "mental clock". Slow practice is for accuracy imo, but it's super useful to be confident that you can play at that speed even if it sounds terrible. I'd say that confidence is a one-stop solution to overcoming mental speed blocks.
All of which is to say, you haven't one-upped me yet! 
(But for real though, I mean what I wrote.)
Yeah, I follow: TBH, I wasn't entirely joking either.
As it has happens, for my "day job" I wear steel-toed shoes, which can be fairly heavy.
But I just learned a few days ago that some (perhaps misguided!) competitive runners use "strength shoes" (some silly term like that) during training.
I'm sure you can guess the idea: you know, train with them, then take them off and sprint in minimal shoes, and one is a superhero, or whatever.
In running, or even walking, that does sound like a recipe for injury, but it is indeed a technique. I don't think it's a great idea, but for someone who knows how to pick the ivories, it's not bad to "feel" the way different tempi.
Certainly in piano, where one doesn't need "raw strength" (stamina, in many places, but that's different).
Here's my latest unpopular opinion: one can't learn (good) piano playing from books. Only two options: learn it by listening, or have somebody show one.
Or both. But, it can't be learned by reading alone. It's an art, you know! More than that, it's very literally manual labor!
I don't think that's an unpopular opinion, but knowing the internet, somebody would disagree!
**EDIT** About the hyper-fast, I think the jazz pianist/pedagogue Hal Galper had a nice ¶ or two about being able to "hear" quickly. In his excellent book,
Forward Motion, which has some good analytic work on how Bach created lines that are relevant to superior jazz lines. IIRC, he was talking about some ridiculous very fast passage, partly or mostly improvised. Perhaps somewhere else he said that.
Together with Bert Ligon,
Connecting Chords With Linear Harmony, one of the very few "jazz" books worth the paper they're printed on, mostly because they don't pretend "jazz theory" is some special "fancy theory": no special system required, just regular musicianship anyone with a good record collection or a few friends nearby can handle. At least in a workmanlike fashion.
If you can hear the "raw velocity," or, if one is Iggy Pop, "raw power," that's really most of the battle, it seems to me.
We can all wiggle our fingers pretty fast, and I think a lot of us can do so efficiently with proper technique (or one of several possible techniques), but what's missing?
Hear quickly, beyond the capacity of singing solfège, and toward the point of precise intuition.
That sounds a bit vague, but I think it's right.
Also, one more unpopular opinion: depending on one's goals, you don't need a teacher to play certain kinds of piano. A record player, an instrument, and ideally a small (or large) community of fellow musicians.
EETA One last unpopular opinion. I actually think the movie
The Hateful Eight is a remarkable movie, in many respects. I don't think it was a popular recent movie, but that's probably due to illiterates who don't know Budd Boetticher movies, or Randolph Scott, or, really, any of that tradition. Video game people, or little earphone heads with dirty necks. Not claiming it's a great movie. But, more or less, people who cannot articulate reasons for their distaste do not have valid opinions.
Not because of ressentiment, but for want of articulate opinions of their own.
Defend, or assert, with reason. Doesn't matter if the reason is right or wrong: there is reason, or that person ceases to exist as a human, except in the most unimportant of appearances.
Rationality is the birthright of humanity, and to act without reason is the province of the fool, the emotionally derelict, or she or he who disregards the ecological niche of the human species.
Topic: Unpopular Opinions