Okay, not having a 100% clear sense of your problem, I rethought the whole thing.
First, I don't yet have a feeling for just when and how extreme are these slowdowns.
Please run Task Manager, and put it on one side of your screen. Click on the Performance tab. Then click on Resource Monitor. Leave that on one side of your screen. Now, run a local app (which doesn't use Internet) which you know will slow down the PC, and watch the Resource Monitor numbers for CPU, Disk, and Memory. Which ones jump the most when you get your slowdown and by how much? You will likely need to do this several times. If necessary, record a video of your doing this.
Next subject: How old are these games? I'm starting to think that half your problem might just be a generally underpowered, outdated PC for gaming. You do know that a 11 year old CPU will not run apps like today's processors, right? Esp. if you are specifying very high resolutions/framerates/texture levels. And you've only got a measly 4GB of RAM. I wouldn't think about gaming on less than 8GB, and preferably 16GB. So there's that. The results you give us from the Resource Monitor test above should give us some clue as to whether lack of RAM is an issue.
Even your hard drive is old, and slow. I had the same model of hard drive many years ago, and it was painfully slow, even though the specs suggest it shouldn't be.
Now, if the problem didn't always occur, that's different. But we need to be clear. If by "didn't always occur", you mean that you started playing more demanding games with heavy graphics/3D etc. then the problem may have always existed. By that I mean that your PC would never have been powerful enough for the task, but you just might not have noticed it until you started playing graphics-intensive games.
As for the network aspect...
Hmm...the UPC Connect Box. That has Gigabit Ethernet, so that likely eliminates the switching aspect of your LAN network in terms of bottleneck (slowdowns).
The Internet service aspect...
"Profile" means Internet user profile, also known as the bandwidth (speed) you're supposed to get on your Internet account. What speed are you paying for? Watching a video on Youtube is an online activity. Again, we really need to know your Internet speed. Whether or not you're getting what you pay for is not as important as are you getting what you need. If two people are trying to do advanced online gaming with a 10Mb connection, good luck with that. When you say "spike", do you actually mean "dip" or slowdown? A spike is generally a short period during which something increases.
You're talking about viewing a Powerpoint presentation. Using what software? Do you have Powerpoint on your PC, or are you viewing something in your web browser? You need to be 100% certain about this. Otherwise, maybe you're doing this online without realizing it.
Keyboard delays can happen due to a bad keyboard or bad keyboard port, or many other types of things that just generally slow the computer. In general, many kinds of things which slow down the CPU can take its attention away from the keyboard, resulting in delayed keypresses.
Edited by Shplad, 29 January 2019 - 05:16 PM.