Well, I benchmarked two sites (elgg 3.3.24 and 4.3.1) and here's what I found.

Elgg 4 makes more database queries under the same conditions than Elgg 3.

Sometimes it increases the load avarage:

Elgg 3: ~0.8.x ~0.2.x ~0.1.x

Elgg 4: ~3.x ~2.x ~1.x

But it always increases the load on the processor:

Elgg 3: ~10%

Elgg 4: ~50%

Enable the logging in PHP (slow log) I found the following slow requests:

preg_match() vendor/matthiasmullie/minify/src/Minify.php:298
replace() vendor/matthiasmullie/minify/src/CSS.php:316
execute() vendor/matthiasmullie/minify/src/Minify.php:149
minify() vendor/elgg/elgg/engine/classes/Elgg/Views/MinifyHandler.php:39
__invoke() vendor/elgg/elgg/engine/classes/Elgg/HandlersService.php:63
call_user_func() vendor/elgg/elgg/engine/classes/Elgg/HandlersService.php:63
call() vendor/elgg/elgg/engine/classes/Elgg/PluginHooksService.php:69
trigger() vendor/elgg/elgg/engine/classes/Elgg/Application/CacheHandler.php:343
getProcessedView() vendor/elgg/elgg/engine/classes/Elgg/Application/CacheHandler.php:124
handleRequest() vendor/elgg/elgg/engine/classes/Elgg/Application.php:422
run() vendor/elgg/elgg/engine/classes/Elgg/Application.php:345
route() vendor/elgg/elgg/engine/classes/Elgg/Application.php:395
index() index.php:8

I'm sure that caching in Elgg 3 works better than in Elgg 4.

I don't have a clear answer as our sites are different, maybe my report will give you the true way and you'll come back having learned the reasons why your site is so slow.

This experience proves that version 3 is faster and consumes fewer resources than version 4. This is strange. I think that updates may contribute to increasing the speed of the site and reducing resource consumption. We aspire to re-improve the performance of the script and work to improve its performance and reduce its consumption of resources

We tried to keep the performance similar between 3.3 and 4.0 (otherwise our client wouldn't accept the upgrade ;).

That said, performance depends on many factors (hardware, network, plugins, etc).

As far as I remember it was quit similar. During the development of Elgg 4 we improved the performance, this had a lot to do with caching, as we switched to a different caching library and some configuration improvement were made.

It is possible that Elgg 4 requires more resources, I don't remember exactly. We try to reduce this as much as possible, for example the number of database queries is something we try to reduce as much as possible.

A tip I can give is check the new settings.php file in Elgg 4 for the added options when compared to Elgg 3. This can help with performance.