How to diagnose Android performance (responsiveness) issues?

Question

Android

1) Please, who is familiar with Android at the OS level, explain how you can diagnose Android performance (responsiveness) problems in order to understand WHAT to fix if there are constant lags in the system?
What software, or where to look in the operating system? What log, and how to interpret? Are there any serious manuals related specifically to the Android performance problem?
From the "amateur" level ("layhacks" in blogs), everything that is possible has already been googled and read. Tips usually come down to cleaning the cache and memory, deleting applications, "freezing" applications that run in the background and wake up on events, limiting the number of background applications to two. And there are more exotic things like setting processor modes (governer) and additional "entropy generation".
A lot of software has been tried for cleaning and optimization, including CCleaner, 3C Toolbox Pro, Android Assistant, Pimp My Rom, Seeder, Greenify, Servicely. There were a lot of experiments with different tweaks, but most often this is done blindly, because the most interesting tweaks are poorly documented.
All this sometimes helped, and it helped very noticeably - the lags disappeared completely, the device flew.
But over time, either with the installation of new applications, or with updates, or with some changes in the system, lags appear again, and all this, there is such a suspicion, has already turned into a situation where the system brakes are no longer caused by the original problem, but my crooked attempts to fix everything.
2) There is a strange problem related to the frequency adjustment of the processor. My CPU is declared with a frequency of 2.26GHz. But the system almost constantly limits the frequency to 1.19GHz (give or take), regardless of the governer and the actual CPU load. That is, in the task manager I see a CPU load of more than 80%, even the interface lags, but the frequencies do not increase. If I specifically set the minimum frequency above 1.19GHz in 3C-Toolbox-Pro, it really does not rise anyway.
Object:
LG Nexus 5 device, Android 5.1.1, rooted.
Data collected by AIDA64
UPD under item 2)
It turned out that the so-called "thermal throttling" is wired in the core of the device, which limits the processor frequency at a certain temperature. According to the experience of many people, this very throttling starts to cut the frequency after about 3 minutes after the device is loaded, without running toys or other "heavy" applications. At the same time, it cannot be said to the touch that the device is very hot. Here, some have a fair question for LG and Google: why put a processor with a frequency of 2.26GHz into the device, if this frequency is almost always cut by half (!), Without the ability to manually adjust even with root rights?
As a solution, they propose to put some kind of update on the kernel - ElementalX Custom kernel, which allegedly allows you to remove "thermal throttling"
https://elementalx.org/
Haven't installed it yet. First you need to update the firmware to a newer version (this thing is not provided for Android 5).