Sorry, I was thinking of Snapdragon 630. Qualcomm needs to not mix their GPU/CPU numbering system.
Testing I found shows the 845 beats the A11 in GPU performance. While the A11 wins in CPU performance. Especially single core.
It's just the emphasis Qualcomm and Apple went for. All that extra RAM (8GB/6GB vs 3GB) in Android phones probably has to do with it too. The Oneplus 6 also appears to hold a marginal advantage over other 845 phones. Reading an Anandtech review suggests a better cooling design. It can sustain near peak speeds when other phones start to throttle. A Tomshardware review also spoke with Oneplus reps who said they did under the hood tweaks so that the phone does waste resources rendering frames that are not seen by the user.
Anyways, the A11 dominates the One Plus 6 in CPU performance. It's way ahead in Geekbench Single Core, Multi-core and compute. It also is way ahead in Octane 2.0 (web browser). Transcoding with Adobe Premiere a two minute clip from 4K to 1080p took the iPhone 47 seconds and the OnePlus 3 Minutes and 45 seconds.
3D Mark has more detailed tests for 3D performance. Of the tests which are presumably the same. I have no idea if the underlying graphics engines are the same. The One Plus 6 beats the iPhone X in all four measurable benchmarks. Ignoring those both models maxed out as there is no way to compare. I would note the One Plus does not appear to be special in these benchmarks. Other 845 phones appear about the same.
https://appleinsider.com/articles/18/05/27/video-iphone-x-vs-oneplus-6---benchmarks
https://benchmarks.ul.com/hardware/phone/OnePlus+6+review
https://benchmarks.ul.com/hardware/phone/Apple+iPhone+X+review
https://www.anandtech.com/show/13038/the-oneplus-6-review/4
https://www.tomsguide.com/us/oneplus-6,review-5438.html
If gaming performance is a big deal for you. You may want to see if you can find any Qualcomm 855 phones in your local market. As the pricing of some are close to the OnePlus 6 at least in the US. Although I'm having trouble finding any which are local models. On that note be careful when buying non-local phones. As many aren't global and make regional models of the same name supporting different bands. They may not support the local bands needed for LTE. You need to look know the exact model then look up the bands that specific variant supports and what your carrier supports.
https://www.techwalls.com/qualcomm-snapdragon-855-smartphones/