Every "hop" between mesh devices reduces the bandwidth by half.
If the device is using a single radio, the available bandwidth is divided by the number of devices communicating at the same time. The number of hops adds latency, but it's the number of devices at the most heavily loaded point that determines speed.
Lets say you have a single 5Ghz wired AP. It has 1 client. That 1 client gets full speed. 2 clients, they each get half speed. 3 clients, 1/3 speed. (Again, assuming they all communicate at the same time.)
Lets say you have an AP and a repeater. The AP has 1 client, the repeater. The repeater has 2 clients and the AP. Assuming 1 radio, the repeater has to split it's bandwidth 3 ways, 1/3 to each.
Lets say you have an AP, a repeater, a repeater, and a repeater with 2 clients. The AP has 1 client, the first repeater has 2 (splitting it's bandwidth in half), the second AP has 2 (again, splitting in half), the third has 3 devices (2 clients, the previous repeater.) So, you're still getting 1/3, because the final repeater is the bottleneck. But you'll never get more than 1/2, because then the other repeaters become the bottleneck.
Now, assume 2 radios, the most common scenario. If the clients always connect on the 2.4 and the repeaters always connect on 5, excluding interference and all that, it works out like this: Repeater 1 with 1 client, full speed, repeater 2,3,4,5,etc. with 1 client each, half speed (because all the repeaters now has to split it's bandwidth between 2 devices). That half speed is divided between all the clients communicating at once of course, so it's possible they could max it out.
Now, assume 3 radios, clients on 2.4, repeaters on two different 5Ghz channels. All repeaters are now at full speed (each radio communicates with only 1 device), same as the AP. (But with more latency.) Repeater 1 uses radio 1 to communicate with the AP, radio 2 to communicate with the next repeater, radio 3 to communicate with clients. Etc. That's how the updated Orbi works if I remember right.
A mesh bumps the complexity up a notch. Assuming 3 radios, the AP can talk to repeater 1 and 2 at full speed (assuming they are in range). Repeater 3 can talk to repeater 1 and 2 at full speed (again, assume range isn't an issue.) Clients on repeater 1 max it out, repeater 3 can now route it's traffic via repeater 2, bypassing the maxed out repeater 1, maintaining full speed. That's what makes a mesh network different from a repeater network.
Wiring a node (repeater) in a mesh network is just giving it another, dedicated, channel to communicate on. Theoretically, if the ethernet was at capacity the node could send some data via ethernet and relay some via other nodes wirelessly load balancing between all it's available communication channels. A mesh network is also supposed to allow rapid switching of a client between nodes so that it's always connected to the node with the best combination of available bandwidth and reception. Not just when moving around, but when load changes on individual nodes. At the other extreme, some meshes are "community" sized, such that if my cable modem is at capacity or goes down, some or all of my data might be routed across several nodes to your cable modem and from there to the internet.
There's even a sort of mesh thing for cellular service. I might not be in range of the tower, but if I'm in range of you, you're phone could relay my call/data to the tower. It'd be particularly useful after disasters, like what Puerto Rico is facing now.
After typing all that, I sure hope I didn't miss some key point and end up being spectacularly wrong, but if so, live and learn I guess