Well you have the same GPU as me and I have had the same problem whilst streaming, but like you said the stream is fine but the game FPS is bad.
There could be a lot of reasons for this, but I'll list a view that make the most sense.

Bottleneck in the graphics card, which means the CPU is weak or just making the performances of the gpu perform badly. Since you have an i7 I doubt that this could be the reason. (I used to have an i3, which was bad. And now have an 3.7Ghz 6 core AMD. And the problem has gone for me)

Another reason could be because the GPU simply isn't just powerful enough to record for the stream upload, and have the game on one monitor and having another monitor with everything else open in. But I have the same GPU as you, but no longer get the problem so I not sure.

The final reason, which I think is the most believable one. If your monitors have a 60hz display and in game you're getting more than 60fps like 150, there can be fps drops because the fps isn't the same as the monitor, and if it does this from 150 to 60 or 90 or something like that, it can cause fps drops that look like lag or dips in performance. If that makes sense, can't really describe it well, but if you have V-sync on (which makes the game go the same hz or fps as the monitor), it will stop the fps from dropping so much and can even fix the problem completely.

I know I probably haven't made a lot of sense, but yh I hope I helped.

If you play on euw and wanna play with me or have more questions, add me (Sp cebar)

zSiiCz wrote Well you have the same GPU as me and I have had the same problem whilst streaming, but like you said the stream is fine but the game FPS is bad.
There could be a lot of reasons for this, but I'll list a view that make the most sense.

Bottleneck in the graphics card, which means the CPU is weak or just making the performances of the gpu perform badly. Since you have an i7 I doubt that this could be the reason. (I used to have an i3, which was bad. And now have an 3.7Ghz 6 core AMD. And the problem has gone for me)

Another reason could be because the GPU simply isn't just powerful enough to record for the stream upload, and have the game on one monitor and having another monitor with everything else open in. But I have the same GPU as you, but no longer get the problem so I not sure.

The final reason, which I think is the most believable one. If your monitors have a 60hz display and in game you're getting more than 60fps like 150, there can be fps drops because the fps isn't the same as the monitor, and if it does this from 150 to 60 or 90 or something like that, it can cause fps drops that look like lag or dips in performance. If that makes sense, can't really describe it well, but if you have V-sync on (which makes the game go the same hz or fps as the monitor), it will stop the fps from dropping so much and can even fix the problem completely.

I know I probably haven't made a lot of sense, but yh I hope I helped.

If you play on euw and wanna play with me or have more questions, add me (Sp cebar)

I get what you mean, I only get around 60fps anyways even not streaming, but what's weird to me is that I only get drops while in borderless... I'll try the vsync though!