Low framerate problem with almost every recent games

Try uninstalling your drivers and use DDU to remove all video configs. Then reinstall and test.
http://www.guru3d.com/files-details/display-driver-uninstaller-download.html

Download and install CPU-Z as well as CPUID HWMonitor. Open CPU-Z, find the tab labelled "Bench" and click on the button that says "Stress CPU". It isn't a full 100% load, but it's enough to give you an idea of how hot your CPU is getting. Let it run in the background for about 15-20 minutes, while looking at the temperatures listed in HWMonitor. There should be a bunch of temperatures, one per core, and one overall. Report back to us with the highest temperatures recorded per core, or post a screenshot. You could use Speccy if you wanted to measure temps, since you already have it.

Tobacco smoke leaves a sticky residue on everything, especially something that sucks in air all the time like a computer. Dust sticks better to sticky things. As a smoker, you're going to have to do wayyy more deep cleaning than the average non-smoker. I was once a smoker, so I know how it goes. Smoke outside, or at least in a different room from the computer. Keep the room well ventilated, so the smoke doesn't just sit around.

There are plenty of good guides on Youtube for how to overclock CPU's. All I can tell you right now is to take it very slow, be patient, and expect it to take between a couple days and a couple weeks. Check twice, change once. I once spent over a half a year perfecting an overclock, changing one tiny setting every day and seeing what changes it made to overall performance and stability. Overclocking is not an exact science. No one can tell you "input these numbers and it'll work 100%". Anyone that tells you they can has a screw loose in their head, or has little experience overclocking. Get comfortable with the clear CMOS procedure. Don't set any expectations. Anything you achieve over stock is already outside normality, so to speak.

You've got a Z77 motherboard, and not a bad one either, so at least you have this option available to you. There's a first time for everything. There's a learning curve, but more knowledge is never a bad thing. Everyone's gotta start from somewhere.

[then maybe you can overclock your graphics card too ]

Download and install CPU-Z as well as CPUID HWMonitor. Open CPU-Z, find the tab labelled "Bench" and click on the button that says "Stress CPU". It isn't a full 100% load, but it's enough to give you an idea of how hot your CPU is getting. Let it run in the background for about 15-20 minutes, while looking at the temperatures listed in HWMonitor. There should be a bunch of temperatures, one per core, and one overall. Report back to us with the highest temperatures recorded per core, or post a screenshot. You could use Speccy if you wanted to measure temps, since you already have it.

Tobacco smoke leaves a sticky residue on everything, especially something that sucks in air all the time like a computer. Dust sticks better to sticky things. As a smoker, you're going to have to do wayyy more deep cleaning than the average non-smoker. I was once a smoker, so I know how it goes. Smoke outside, or at least in a different room from the computer. Keep the room well ventilated, so the smoke doesn't just sit around.

There are plenty of good guides on Youtube for how to overclock CPU's. All I can tell you right now is to take it very slow, be patient, and expect it to take between a couple days and a couple weeks. Check twice, change once. I once spent over a half a year perfecting an overclock, changing one tiny setting every day and seeing what changes it made to overall performance and stability. Overclocking is not an exact science. No one can tell you "input these numbers and it'll work 100%". Anyone that tells you they can has a screw loose in their head, or has little experience overclocking. Get comfortable with the clear CMOS procedure. Don't set any expectations. Anything you achieve over stock is already outside normality, so to speak.

You've got a Z77 motherboard, and not a bad one either, so at least you have this option available to you. There's a first time for everything. There's a learning curve, but more knowledge is never a bad thing. Everyone's gotta start from somewhere.

[then maybe you can overclock your graphics card too ]

Those temperatures look good. Really good. Cleaning it definitely helped. Good to see the situation is improving. Did you clean out the rest of your computer too, like the power supply and graphics card?

Now for the real test: see if playing games isn't still slow, choppy, and bad, especially Battlefield 1. Fallout 4 isn't a well-optimized game, and Hitman is horribly unoptimized. Leave HWMonitor running to record your highest temperatures, and go have some fun.

Try what elbert said about Nvidia drivers. I know nothing about Nvidia drivers, since I've owned AMD graphics cards ever since I've been toying with and building PC's, let alone Macs.

Surprisingly, you're probably at about the limit of your GTX970, especially in BF1 if you've turned up your graphics pretty high. It takes quite a more powerful graphics card (GTX1060, GTX980ti, or better) to sort of max out the graphics settings in BF1 at good framerates (60fps+) at 1080p. A GTX1070 is more suited to that job, when paired with a 6600k or better. Ultra in BF1 at good framerates at 1080p is just not possible with the GTX970.

You're also at the limit of your CPU. If you don't want to be a huge nerd and spend a couple days in and out of the bios getting a stable overclock dialed in, I suggest you go buy some new parts. If you don't want to put the work in, the next best thing is to throw money at the problem. Really. You can't really get much better a CPU for your current motherboard, so that means buying a brand new CPU, RAM, motherboard. At that point, may as well just build an entirely new computer, and sell the old one.

What you're experiencing is a CPU bottleneck. Smoke, explosions, particle effects, shadows, fog volme, fire, trees, grass, etc. are typically exclusive processes to the CPU. When there's lots of it, your CPU gets hit so hard, it gets backed up, and everything slows down. The same applies to large numbers of AI-driven NPC's in games like Hitman and Fallout 4. Mafia 3 in generalreally needs a beastly CPU. In multiplayer BF1, like every other MMO and online game of its kind, the CPU processes all the information coming in from the server, as fast as it possibly can, including information like other players position, bullet and projectile trajectory, hit detection, other players destroying the environment, the behemoth, vehicles, explosions affect radius, etc.

What you choose to do, is up to you. All we can do is make suggestions.