Colemak and pretty much all alternative layouts have huge advantages over QWERTY.
Colemak is not optimized for hand alteration, but it still has better hand alteration than QWERTY.
There is a large debate over whether hand alteration or rolls are more efficient, and some studies such as MTGAP have theorized that the most efficient trigraphs are actually ones where 2 letters are rolled on one hand and the other letter is on the opposite hand.
Honestly, keyboard layout optimization is a deep rabbit hole where it seems obvious at first glance what is more or less efficient, but the more you study it, the more complicated it gets.
Different layouts all try to optimize different aspects. Dvorak has great hand alteration, but higher same finger repeats, loses the most of the convenient ctrl editing short cuts, as well as overloading the right pinky with high frequency letters. When you try to optimize in one area, you end up sacrificing in another. Which area should be weighed more heavily is always up to the user.
I mean even the main premise of putting better keys on the home position is up for debate and nobody actually agrees on what the home position really should be.
For example, Workman layout tries to account for the way fingers actually work (middle fingers are longer and reach the top row easier but has trouble curling, index finger is short but can curl and hit the bottom row easier, etc.) Thus the middle rows where H and G live are considered poor positions in this layout but prime positions in most other layouts.
If we really want to talk about optimized, we should really be looking at layouts like Balance 12, HIEAMTSRN, and the different versions of MTGAP. These layouts have low finger repeat and good hand alteration as well as more optimized punctuation but they require a lot of pinky usage.
Here is a site that lets you compare metrics on a ton of layouts as well as custom ones vs any text you want:
http://patorjk.com/keyboard-layout-analyzer/#/mainJust make sure you ignore their rankings because rankings are totally based on how you weigh the different attributes.
And here is a site that lets you try different layouts in the browser:
http://blog.mikekuehn.ca/keyboard-layouts/I spent an entire summer researching and playing with layouts so ask me any questions you have.
I'll just tell you right now though that the answer is usually "depends on your personal preference."
PS. Speed is usually not something you gain too much of with a new layout if you're already good at QWERTY. What you gain is a more comfortable typing experience that doesn't tire your hand out as much and helps prevent RSI.