I have the same issue and some more details:
1. @PšendaUsing multiple monitors isn't a "big load", it is a marginally higher load than using one monitor and can pretty much be compared to using a higher resolution monitor, with today's hardware even on most laptops, any issues arising from using two monitors in applications that don't have a constant high performance demand is usually a software bug and not an actual performance problem.
2. @Sean PI have tracked down what is causing these issue for me: Having the "History" studio window visible causes medium amount of typing latency, having the "Appearance" window open causes high added latency, having both visible stacks and causes a very high amount of latency from key press to showing up on screen.
My relevant specs are:
OS: Windows 10 1909
Software: Affinity Designer 1.8.3.641
CPU: Intel Xeon E5 2630v3 with unlocked 3.2GHz boost.
GPU: Nvidia 980ti
Disk: Affinity is installed on an SSD that isn't showing any errors or corruption.
Screens: 3440x1440 at 120hz and 1920x1080 at 60hz - I tested for this behavior with only the 1920x1080 at 60hz screen connected, which should be "the most standard" display configuration and it was still present.
Here is a video of the issue: https://i.imgur.com/8JBx0y4.mp4 I can't really show when I am actually pressing keys so when you see the mouse starts moving is when my hands are off the keyboard.
