I've recently upgraded my phone from a Samsung Galaxy S6 to S8.

The S6 always ran with CPU time very similar to runtime but the S8 runs for 58-60,000 seconds to complete the usual 28,800secs of CPU time. How do I change this? I can't see any way to adjust settings in the App - and I'm not sure what settings I'd change either tbh.

The S6 is currently only set to run 1 task at a time while the S8 runs 4 at a time.
S6 runs Android version 7.0, S8 runs v8.0.0
S8 settings:
Boinc CPU limit 100%, pauses at 80%
RAM limit 70%

Preferences>CPU>Used CPU cores
I suspect that if you reduce the number of cores to one or two, you will get the two numbers much closer to each other. Probably an i/o bottleneck with four cores running. (May also be a temperature thing. My phone gets too hot with all four cores crunching and keeps shutting off to cool down. Also it hammers the battery so I have gone for computing only when on USB power.

You cannot compare the work load on the CPUs on an Android device with that of, for instance a personal computer. The octa-core Android devices usually have 4 big cores for intensive workloads and four LITTLE cores for lighter tasks. These latter cores run at a slower speed and are normally used by BOINC and the science applications. The faster higher speed CPUs aren't used by BOINC or the science applications, no matter whether you set BOINC to use all cores or not. (See Github Issue #2549 for the discussion on this.)

That's probably done by Android design and may not be possible to be overridden.
When you now set BOINC to use all cores, only the slower LITTLE cores are filled, so when there's 8 cores it means these cores run two tasks per core.
Best set BOINC to use 4 cores in that case.