I have noticed this about Blender if you want to unwrap a very high poly mesh like after you click smart uv project, it takes hours to unwrap. I have a mesh that has taken over 4 hours and I am still waiting for the unwrap to finish. 
Because the algorithm is not linear… double amount of polygons does not take double time…
It’s not the same but imagine:
- 10x10 vertices ends up in a 100 vertex grid
- 100x100 vertices ends up in a 10000 vertex grid
- 1000x1000 vertices ends up in a one million vertex grid
…something similar happens while unwrapping : the algorithm checks against a lot of vertices if they are a neighbour and in what angle the are and somehow tries to lay them down into 2D space…
…and the more you have…
It would finish far far faster if you added seams in yourself.
From memory, it is the packing the islands into the UV map that takes lots of time.
Well for complex hard surface models, it will take a long time anyway. a full car model can take all day, if you include all the little doo dads. I Try and sort out the major shapes in the uv layout first, pin those uv island then keep on repacking. Every island that is finished I pin it, then pack the remaining islands
For example I have a lorry cabin with window insets and various things going on, so I use project by cubic to get the main shapes, then adjust the seam, stitch where required
sometimes it hard to know where to place the seam to get minium distortion, so using cubic projection first, gives you a good start
Also its nice to make the UV map readable, so you can recognise the parts from the layout
UE5 works totally different. Blender is modelling program (and much more), UE5 is not.
First of all: blender are very bad for unvrapping, and no, addons doesn’t solve this.
Blender unwrapping are slow, but even worst - in tons of cases something which would be unwrapped without any problem in dedicated software (RizomUV as example) will lead to pile of mess in blender.
Even RizomUV would struggle with highpoly meshes (depends on polycount, of course)
Second: normally no one unwrapping high-poly meshes. If model was made for subd - you unwrap “low-poly” and after subd uv will be updated.
If you dealing with auto-retopologized/decimated high-poly sculpts - when you would be needed to do manual retopo and unwrap after this.
Even few minutes of unwrapping (smart-projection, etc, even in blender) are clearly indicate what you doing something wrong, i.e - you try to unwrap something too high-poly.
I was convinced that smart UV was a python script, but it turns out it was ported to C a few years ago :
https://archive.blender.org/developer/D8311
While it’s already a great improvement, that doesn’t mean the code is optimized for complex mesh.
I suspect that if you unwrap your mesh into chunk and eventually pack everything you’ll get a more manageable workflow.
Until someone jumps in and tackle some optimization.
And of course, it’s hard to compete with dedicated software for unwrap, that might be simpler if you have a lot of highres mesh to do…
Well… RizomUV… is especially developed to unwrap… and only unwrap… so it would be a bit weird to pay 35 to 60 EUR for a software which does it not better than blender… 
RizomUV are miles ahead of blender and always would be. Im only say what “Even RizomUV would struggle with highpoly meshes (depends on polycount, of course)”. RizomUV will still provid few times more speed for unwrapping high-poly meshes, but no one told its a magic and you can unwrap few million polygon sculpt.
If you now relativise like this:
Then i do not understand your argumentation before:
What are you trying to say… ?? Explaining the mathematical founded reason why it’s imposible to map 2D to 3D as a bijective function… or moaning how bad blender is… 
What’s your point. ??
How about reading the whole messages, because its right there:
- First of all: blender are very bad for unvrapping
- Even RizomUV would struggle with highpoly meshes
- Even few minutes of unwrapping (smart-projection, etc, even in blender) are clearly indicate what you doing something wrong
I just not getting your point when you start with: “blender is bad in…”, “even with any addon…” and then mentioning a better suited app… and lastly also saying that even this woudl struggle…
…it’s simply (even in general) :
"The user is not supposed to use any automatic feature to a very complex object, because (s)he has to understand the underlaying problem first. This said: (s)he may use this for a specific area with accordingly choosen parameters ( using the knowledge about the (un-)possibilties ).
Maybe the question is somekind of vague because the user doesn’t mention any experience with other programs… but the question itself leads to the assumption that the OP just doesn’t know better… so nothing what you said will really help to clearify this… and especially not:
…well… that’s why he asked…