Resolution drop when using DP to HDMI adapter

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Your post sounds quiet incomplete, lacking much of context information. See the community field guide on posting good questions, including its references close to the end.

spicehead-oehl3 wrote:

Why?

Probably because you've chosen the wrong adapter. Try using an adapter meeting your requirements not just the physical interfaces. Did you mean that you don't know your requirements? We don't know neither unless you disclose them. More context information might help to assist you.

To which protocol version of HDMI at which resolution and which image rate do you refer? To which protocol version of DP at which supported resolutions and image rates do your refer? What are the requirements of your VR device of which make and model? Which resolutions and image rates does your graphics card support of which make, model and memory? What are the limitations of your HDMI cable of which length?

I don't have any VR equipment but I also use a TV monitor to display some computer information in some context sometimes at up to UHD resolution with 60 images per second as specified in HDMI 2.0. None of the computers getting sometimes connected has a dedicated graphics card (GPU), only integrated GPUs of the CPUs. Some of them even support two displays at such a resolution but didn't test so. I use them also in dual-screen setting often without issues. Some of them don't have an HDMI port while having either a USB-C port or a DP port. I successfully tried to find the specs of my equipment before selecting adapter and cable to connect. For some devices even this wasn't sufficient and I needed to update the GPU driver and firmware to get this setup working as promised by the manufacturer in their product description.

Are you sure that your VR equipment doesn't support DP as you claim needing HDMI for your VR equipment? Depending on your DP and your HDMI, HDMI might be more limited than DP. That's at least what I read in the specs of one of my computers. And to find such specs can often be tricky as these are often hidden in the manufacturer docu. For some computers, I couldn't find it at all and only realize by transposition of other devices. For most computers, information supplied and delivered with equipment was incomplete but included references to online docu which had the info I was looking for. And I can imagine that your TV monitor isn't an appropriate component for your VR equipment for VR experience. You may nevertheless succeed in using it for VR application but probably whithout getting a VR experience. And if you didn't lookup the specs of your equipment including your VR equipment, you should do so first as you'll learn much more restictions and constraints specified.