Apple is dumping Intel processors and going ARM. (An Apple Store employee prognosticated this to me in late 2018). Last two times they did this, they badly burned people who had just bought 68k or PPC Macs. They're handling it better this time by letting it be an open secret, but I would not buy a new Mac for now.
I'm writing this on a mid-2007 iMac right now, so obviously I'm OK with older hardware.
When your system starts to bog, your first stop is Applications/Utilities/Activity Monitor. It has several panes that will tell you about system health, and that will tell you what you can (or can't) do.
Leave Activity Monitor running. When the system is bogging, switch tasks to Activity Monitor and see what it can tell you. It has several "panes" which you can switch between; mind you that if you switch away from a pane for 10 minutes or so, it stops monitoring that system parameter (because that monitoring itself adds some load). So if you're starting something you expect to bog, it helps to pick a pane and have it up while the boggy task is running.
The CPU pane
will show you how hard your CPUs are working. That's pretty straightforward. If all your CPUs are maxed, that's not upgradeable, so your only path is a newer Mac. If one CPU is maxed, the process is running single-threaded, and sometimes you can do something about that.
This example shows a process that is mostly CPU-bound but slightly diskbound also (see disk example). Note that on my 2-CPU system, when it uses 50% of the chart height, it is at 100% for its single CPU.
The Memory pane
This is very important (whether you're able to upgrade or not). It shows an abstracted chart called "Memory Pressure" - that's because memory management is tricky - it doesn't just do disk swap, it also will also use ZIP/gz-style compression to compress RAM that is not used in the moment. "Memory pressure" reflects how much both strategies are affecting performance, with a simple green/yellow/red display.
In yellow/red, you will feel the lag. The upgrade path depends on what is taking memory (the task list above will state memory use). If they are other applications you don't care much about, you can close the windows or quit the apps. Watch out for apps with "memory leaks" - these take an outsize amount of memory for what they are. In fact, many apps will take a great deal less memory if you save work, close and reopen them -- this applies to many web browsers/pages.
If that's not enough, and the core apps you want to run are taking that much memory, then your path is a RAM upgrade.
In this example I kept launching apps until the system started to bog. I wasn't closing anything; the downward ticks were memory compression doing its thing. At yellow I was feeling it! At red it was hard to get the system to respond. Force-quitting Photoshop brought the system quickly back to green.
The Energy pane
It's not a concern to a non-battery Mac. It's for Macbooks, and estimates how much battery an app or browser pane is using.
The Disk pane
Reflects disk I/O. It doesn't show a "0-100%" scale because it doesn't know how fast your drives are. The display scales to the highest input and output (separately) you've seen lately.
Look at the "Data read/written/sec" numbers. You just have to learn what "normal" values are for your various drives. For instance I have some external drives that run around 20-25 MB/s when reading large files, so when I see those kinds of numbers from those drives, I know they are near limits, and my process is probably diskbound (limited by disk I/O).
On a hard drive, much of the delay is actually "seek time" - moving the head or waiting for the disk sector to come around. That's much more a problem for "many small files" than "few large files", since hard drives are optimized for large data pushes. Since you already run SSD, you have already eliminated that, so the only upgrade path is a throughput-faster SSD.
Here you can see a process suddenly max out an external drive. It's the same time as what you see on the "CPU" example (but not the "Memory" example). Notice the slight drop in CPU usage just as disk maxed out; that was the process shifting from CPU-bound to disk-bound.
The Network pane
Is telling you about LAN or Internet activity. It's usually not a big concern when you are chasing performance issues, since you expect the Internet to be slow/capricious.