Inspiron 7559 - Extremely slow startup

I have some trouble with booting my Inspiron 15 7559 (specs below). The problem occured like two days ago.

The main problem is that booting up since the power button was pressed to Windows Desktop last more than a minute. Just for comparison week ago it was not more than 20 seconds!

After everyting is loaded and I am finally inside Windows, I've not discovered any high CPU, RAM or HDD usage.

What happend
I've been copying files from one folder to another (from C:/ to D:/ drive), but the copying started to be extremely slow ( like 1mb/s ) after some time and the whole PC was lagging. I wasn't even able to open Task Manager so I was forced to "hard" restart by pressing and holding power button.

After that I was not able to boot up my system, where I was stuck in Dell welcome screen and the loading wheel was spinning forever...
For no reason I was able to run system into the desktop normally, but everything was very slow for no reason.

What I have tried without success:

• Factory reset ( keeping files, but it actually removed almost everything )
• Full antivirus scan, anti-malware scan - all clear
• Defragmenting both drives
• Disc scan (sfc /scannow) - without problems
• Startup services and programs were optimized (nothing is here what wasnt before, when the system was relatively fast)
• Setting up virtual memory
• Advanced boot options set to 2 processor and memory to 8192MB
• Disabled Intel SpeedStep

Problems:

• Slow system boot (when I enter my account password on login screen, it takes more than 30 seconds untill it takes me to Windows desktop!!)
• Browser lagging - when more tabs are opened the active tab freezing for a moment

I'm very disatisfied right now, since my laptop priced at avg. 1000€ booting up slower than mom's which costed around 350€...

Specs:
Dell Inspiron 15 7559
Intel Core i7-6700HQ
8GB RAM
NVidia GeForce GTX 960M 4GB
1TB SATA (5.4km rpm) 2.5 "Hybrid with 8GB Flash
Windows 10

I'll provide any other informations if neccessary.

Thanks for all answers.