Alienware 13R2 and Fuji X-T2 preview problem

Alienware 13 R2 OLED 13.3 QHD Touch / i7-6500U / 4GB GTX 965M / 8GB RAM / 256GB SSD

I placed clip on timeline (h264, 100Mbits) and preview is very slow unusable. What to do?

Not going to work on your machine.
Resolve needs proper workstation for UHD h264 or slower one but with current Nvidia GPU+ Studio license to have GPU h26 decoding.
Other option- convert to Cineform or DNxHR.

It may work with Resolve Studio with your GPU- not sure.

Generate Optimized Media does not help much.

Look at CPU load. If it's at 50%+ not much can be done. Cineform is probably the best codec in your case.
i7-6500U is just a 2 cores CPU, so 4K not going to work.

Make sure your internal intel graphics are not available to resolve. On my 15r3 i was getting 3fps on gh510bit. I experimented with disabling and then uninstalling the intel drivers which worked temporarily but they always re-instated. I ended up adding resolve to the programs in my nvidia control panel and manually chose to only use the nvidia GPU for cuda. I also unchecked "Use display GPU to compute". I now get real time playback. Hope that helps.

James advice solves problem with "Generate Optimized Media" function.

You should be able simply set Nvidia GPU in Resolve preferences as computing GPU.
Issue may be that Resolve doesn't see it as WIN adaptive GPU engine does not expose it to Resolve.
This may also depend on particular laptop design. Forcing use of Nvidia GPU (through Nvidia profiles) sounds like a good solution.

On my Mac Resolve picks AMD GPU over Intel one by itself (Intel one is not even exposed in Resolve preferences).
Moment you start Resolve forces AMD GPU to be the main one (where screen is connected to), where when you just browse internet etc Intel one is used. Looks like auto GPU switching is better designed on Mac.

I assume your optimise media are half resolution?