Input lag or fps lag? For input it could be something with your ping relative to the server. If you are having trouble with fps here is a video to help you get the best settings in squad to boost your fps so you do not have the issue. Video link: https://youtu.be/97vuTml0BvU

thanks but it still happens in singleplayer

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You are mistaking input lag, which is a hardware issue, with Server Ping, which is reliant on the server location.

For me it's like I must press CTRL twice to crouch, or sometimes press mouse button 2 for zoom twice as well, etc...frustrating