When your system starts getting laggy, take a look at your Task Manager and see what your resource utilization is. HWiNFO64 is good for this purpose too.
See what your CPU, RAM, HDD, and GPU utilization are. If your RAM utilization is only at 50% - for example - than that's not the problem. Upgrading your RAM to 16 GB won't help if you're only using 50% of 8 GB (for example).
In Task Manager, you'll the values at the instantaneous point in time (i.e. right now), but you'll also see a graph over time. The graph is possibly more important, because what matters is which resource your system has been consistently using for several minutes. A spike in CPU utilization to 99% for a second doesn't matter, but being consistently at 99% CPU utilization for 5 minutes straight is something else.
I'm also attaching a screenshot of HWiNFO64 with some of the important values circled. Notice the fact that it keeps track of minimum, maximum, and average values too, not just current. So click the clock at the bottom of the screen to reset those values, and then start running the program(s) that make your computer laggy. Run those programs for a few minutes, and then see what the min, max, and average values are.
Note that in my version of HWiNFO64, I've customized the layout, so your screen will look different. But all the same values and options should be there, just in different places.