Excellent news and thank you haha! 
Good spot on the updated package. Agreed, I should look into updating my rocm packages.
When I have the time to upgrade and test my rocm packages I’ll report back my Resolve performance which should help point us in the right direction for troubleshooting the strange performance of Resolve Studio on your machine.
In the meantime, my suggestion would be to test the performance of the standard edition of Resolve. This will help determine if the problem is isolated to Resolve Studio / sailing the high seas 
.
Less media encoding types are supported by the standard edition of Resolve on Linux. You can use this command to transcode your media to DNxHD, a format which is supported by Resolve on Linux:
ffmpeg -i 'input file.mp4' -c:v dnxhd -profile:v dnxhr_sq -c:a pcm_s16le 'output file.mov'
I recommend transcoding short files which are no more than a few minutes long for the purposes of this test as DNxHD files can be huge. For example, a 2.5 minute 44MB vp9 webm video on my machine increased 50x in size to 2.2GB after transcoding it to DNxHD.