I had this occurring a couple of times in the past, especially the first time was very bad.
I did a presentation for a game jam of a pinball game I made and connecting to the TV via HDMI, my pinball was playing super slow.
At that time, talking to other developers I discover that sometimes it might happens, maybe depending on the device. Though it's the first time I have noticed this with this monitor and I have it for 3-4 years now.
Anyway, I am glad the issue is on my side and not yours, that means that Construct works fine :)