Let's start with what a "Y" processor is and isn't.
Intel's ultra-low power chips used to be labeled "Atom," and had a completely different architecture to its desktop and laptop processors. That didn't go very well for various reasons, so Intel gave up on Atom. To replace it, Intel essentially took the "U-series" chips that were wildly successful in the MacBook Air and Windows Ultrabooks, slowed and trimmed them down, and created a new category of processors. They're built to sip power when compared to the U-series chips, or rather, they're built to not produce much heat, and Intel achieves that by limiting power draw.
A quick aside: Depending on the chip, Intel either refers to these processors as Core M or just throws a Y somewhere in their model number. This is infuriating. They all used to be called Core M, and so I'm just going to call them Core M for simplicity.
The chips in the entry-level MacBook trio are pretty similar. All have the same basic CPU architecture, with equal core/thread counts (2 and 4, respectively) and cache sizes. They diverge slightly in featuresets, and the U-series chip in the dual-core MacBook Pro pairs with faster, less power-efficient RAM, but the main differences you'll find are in clock speeds and integrated graphics.
The 12-inch MacBook comes with an m3-7Y32 in its $1,299 base configuration, or an i5-7Y54 chip at $1,399. Despite the "m3" and "i5" prefixes, these are basically the same chip, tuned differently. Intel even lists them at the same suggested price on its website. The m3 has a base frequency of 1.1GHz and a max turbo frequency of 3GHz, the i5 has a base frequency of 1.2GHz, and a max turbo frequency of 3.2GHz. While I don't have two machines in front of me to test this theory, the i5 will likely increase clock speed more aggressively, but beyond that they are very similar.
The new $1,199 MacBook Air hasn't been dissected yet, but it almost certainly has the i5-8210Y inside. Also a Core M, it has a base frequency of 1.6GHz, and a max turbo frequency of 3.6GHz. It appears to be a variant of the 8200Y we've seen in convertibles like the Dell XPS 13 2-in-1. Where the Air differs from both the MacBook and the XPS is in its cooling: It appears to have a fan, and this would make it a much more capable machine. In fact, it pushes it closer in performance terms to the third laptop in Apple's lineup.
That laptop is the entry-level MacBook Pro. The $1,299 version has an i5-7360U, which has the same turbo frequency of 3.6GHz as the new Air, but runs at a base frequency of 2.3GHz. It also has Iris Plus Graphics 640, while all the chips I've mentioned till now have had UHD Graphics. Without going into too much detail, UHD Graphics are really just concerned with making sure things run smoothly, aiding in things like video decoding, while the Iris Plus Graphics will assist with multiple monitor setups and even let you play something light like Dota 2 on low settings.
So, on paper, all the CPUs are dual-cores capable of hitting speeds between 3GHz and 3.6GHz. But in reality, the three implementations are quite different. Intel has a helpful, but often misunderstood measurement to help us understand why: Thermal Design Power (TDP). Rather than being a measure of how powerful a processor is, TDP tells us how much heat it dissipates at a certain frequency.
All Core M chips are rated for almost identical TDPs at the same frequencies. Idling at 600MHz, they'll likely dissipate around 3.5W of heat, at 1.1-1.3GHz they reach 4.5W and at 1.6GHz they're all around 7W. Unfortunately, Intel doesn't publish Core M TDP figures above that, but the dual-core U-series in the MacBook Pro dissipates 15W at 2.3GHz, and given the architectural similarities, the Core M chips are likely in the same ballpark.
TDP is mainly useful for the manufacturers Intel sells to: It helps them to pick the right part for their device. Apple, for example, knows that if it throws a chip that dissipates 7W of heat in a chassis that can easily shed 7W of heat, it doesn't need a fan, and that's what it did with the 12-inch MacBook.
Now, everyday computing throws some curve balls -- moments when you need extra power to prevent a machine from stalling -- and that's why boost clocks exist. They let a chip kick up to higher speeds for as long as it can stay cool, before dropping the frequency back down. A desktop PC with a good cooler can sustain a high boost clock indefinitely. Outside of gaming laptops with giant vents, portable machines usually can't. It all depends on the cooling system the manufacturer has implemented.
When a system has no fan, though, it has a fixed limit for heat dissipation. Because of this, the 12-inch MacBook, for example, isn't actually capable of boosting to its advertised top clock in everyday operation. The lack of a fan also means ambient temperature plays an important role in performance. As I discovered at SXSW back in March, working in hot Texan sunlight can seriously degrade performance. That's because, unless you're doing literally nothing, the i5 MacBook's CPU won't be at its 1.2GHz base clock: It's almost always hovering above 1.5GHz during regular operation.