Bad performance - clueless gamer

Your build is good and i don't think it's due to the hardware. My Haswell build (full specs in my sig) has a bit lower performance than your build (CPU: i5-4670 vs i5-4590, GPU: GTX 1060 vs GTX 760), but i can play my games on it with high/ultra settings @ 1080p with solid 60+ FPS with ease.

My guess is that you have some applications running that hog your system resources. To know which applications hog your resources, use Process Explorer,
link: https://technet.microsoft.com/en-us/sysinternals/processexplorer.aspx

Just to see how much difference the running background applications makes on system performance, i did some benches about a month ago. Here's two benches of my Skylake build (full specs in my sig),
background CPU usage 10%: http://www.userbenchmark.com/UserRun/3416525
background CPU usage 0%: http://www.userbenchmark.com/UserRun/3416648

Without some background applications running (e.g MSI Gaming App, NZXT CAM, Corsair CUE, Skype, Firefox), the gain is small. Only 4% in gaming and desktop and 2% in workstation. If my CPU would've been used up more (e.g 32%, instead of 10%) the gain in performance would've been bigger.

You can download the Userbencmark tool and let it test your system as well. It's free and a good source to determinate which of your components are under performing,
link: http://www.userbenchmark.com/Software

Your applications seem fine to me. There's no system resource hoggers that i can see.

In your 2nd run, your system had 0,5GB less free RAM and your CPU did run 300 Mhz slower than in your 1st run.

Your CPU is way under performing. While the base clock is 3.4 Ghz, during benching, your CPU turbo was at 2,6 Ghz (1st run) and at 2,3 Ghz (2nd run). Normal turbo speed for i5-4670 is 3,6 Ghz.

What are your CPU temps? Idle and during gaming/benching? Also, can you confirm the CPU speed for me? Also at idle and during gaming/benching.
For those two, you can use the same programs i'm using: CoreTemp and Speccy.
Speccy download: https://www.piriform.com/speccy/download
CoreTemp download: http://www.alcpu.com/CoreTemp/

In CoreTemp, look for the Frequency value and in Speccy, open CPU tab, expand "Cores" and look for Core Speed and Multiplier.
(For system monitoring, i prefer to use more than 1 program, so i can validate the readings between different programs.)

Your PC uptime (15 days) may also be part of the reason. When was the last time you did a reboot? Also, shut down your PC for about 30mins and then power it on again. Look if there's a difference.

Pics about your case and it's internals would help.

Even though Intel's stock CPU cooler is pretty weak, with better PC case, you can have better cooling as well.
Two examples:
1. My old AMD build (full specs in my sig) did came with Codegen ATX-308-CA-USB/Audio (silver/black) case when i bought it as prebuilt office PC. CPU has AMD stock CPU cooler on it and my Codegen case had only 1x 80mm rear exhaust fan. Since i managed to overheat my CPU several times (highest temp that i saw was 102°C before PC shut itself down), i had enough of that crappy case and i bought Chieftec DF-02B-U3 case that has: 1x 120mm front intake, 1x 120mm bottom intake, 2x 120mm side intake and 1x 120mm rear exhaust. After that, my CPU temps dropped like 20°C all around. Now, my AMD CPU idles around 40°C while under load, it can hit up to 80°C.
In total, i have 5x 120mm case fans.

2. As i already stated, my Haswell build with i5-4590 also has Intel's stock CPU cooler on it. But since the PC case is Gigabyte GZ-G2 Plus with: 2x 120mm front intake, 2x 120mm side intake, 1x 120mm top exhaust and 1x 120mm rear exhaust fans, my CPU idles at around 37°C.
In total, i have 6x 120mm case fans.

Bonus: My Skylake build sits in Corsair 760T full-tower ATX case that has: 2x 140mm front intake, 1x 120mm bottom intake, 3x 140mm top exhaust and 1x 140mm rear exhaust fans.
In total, i have 7x case fans, 6x 140mm and 1x 120mm.

If you were to buy yourself a new case with far better airflow, your CPU temps will drop considerably and you can avoid CPU overheating without replacing CPU cooler. Since choosing a case is personal choice, it's hard for me to suggest which case to go for. E.g plain one, fancy one, with/without 5.25" external bays etc.

I'm planning to switch out my AMD and Haswell PC cases, for better airflow, side window and the need for 5.25" external bays.
For AMD build, i'm thinking to get Thermaltake Versa H35 which has ample air cooling with up to 8 case fans: 2x 120/140mm front intake, 2x 120mm bottom intake, 3x 120/140mm top exhaust, 1x 120mm rear exhaust,
specs: http://www.thermaltake.com/products-model.aspx?id=C_00002474

And for Haswell build, i'm thinking to get Corsair 450D with Demciflex filter kit for dust management. (It's far easier to clean dust filters than cleaning dust from inside the case. My Skylake build's 760T already has Demciflex filter kit.) 450D also has optimal air flow with 7 to 8 fans: 2x 120/140mm front intake, 2x 120mm bottom intake, 3x 120mm or 2x 140mm top exhaust, 1x 120mm rear exhaust.
specs: http://www.corsair.com/en-eu/obsidian-series-450d-mid-tower-pc-case
filter kit: http://www.demcifilter.com/p0602/Corsair-Obsidian-450D-Dust-Filter-Kit.aspx

If you can't make up your mind about PC cases or if you like the two above cases, for better airflow, you can go for the same PC cases that i'm planning to go with,
pcpp: https://pcpartpicker.com/products/compare/9JvRsY,36M323/

While Corsair 450D is solid case to go for, the Hyper 212 Evo CPU cooler has weak performance. Though, it still is better than Intel's stock CPU cooler.
I dislike Hyper 212 Evo because it uses sleeve bearing fan. And from here you can read why i don't like sleeve bearing fans,
link: http://www.gamersnexus.net/guides/779-computer-case-fan-bearing-differences

Though, if you would've said that you also plan to get new CPU cooler, i would've suggested something better for you.
E.g any medium sized CPU cooler from this list will do,
link: http://www.tomshardware.com/reviews/best-cpu-coolers,4181.html

The list above also has my i5-6600K CPU cooler, an Arctic Freezer i32, as being the 2nd best mid-sized CPU air cooler. Arctic Freezer i32 comes with semi-passive fan that has fluid dynamic bearing. If you did read about the fan bearings then you can guess why i bought Arctic Freezer i32 for my Skylake build.

With new case and CPU cooler, your CPU will be much better cooled and you can resume your gaming.

Here's also Airflow 101 article for you to read,
link: http://www.tomshardware.co.uk/faq/id-1858957/airflow-101-setting-fans-keeping-computer-cool.html