Hello, so I am facing the issue mentioned in the title. I recently changed my old computer with a newer one but for some reason, this problems appears. In games, however, there is no lag or fps drop. For now I've tested League of legends (I get around 200 fps with very high settings and 80-90 fps in Rise of the Tomb Raider, again, high settings). But then when I play a 1080p Youtubue video, I can just see the lag, same goes for 4K. Videos that are on my hard drive lag as well, even a 720p movie was lagging. I believe everything is updated, I have Flash Player on Firefox. I downloaded a program called SVP that boosts the fps up to 60 and that one 720p movie I mentioned before was actually running smoothly with it. BUT that doesn't really solve the whole issue, right. At first i thought it might be the cable I'm using to connect the PC to the monitor - it's an HDMI one but I'm not sure if it's 2.0 and I know that older HDMI cables might support lower fps. I borrowed a VGA cable with a DVI connector from a friend (since I don't even have a VGA slot on the PC) but the problem persists.
Here are my specs (I haven't overclocked anything):
Windows 10 Pro
MSI X370 Krait Gaming
AMD RYZEN 5 1500X 4-Core 3.5 GHz (3.7 GHz Turbo)
DDR4 16 GB 2667 MHz
GTX 970 4GB GDDR5
The monitor is ACER R241Ybmid IPS.