The H3 is a refreshingly inflexible chip; that's why EB runs so well on so many different boards. AFAIK the only thing that can affect performance on a board level is the CPU voltage it is fed. And that doesn't directly affect performance either, it just limits the clock speeds at which the CPU can be run.
In my experience, the (or rather, my) OPi's run stably up to about 1100 MHz with the default voltages. Other boards may drive the CPU at different voltages and may therefore be able to sustain higher or lower maximum clock speeds.
The CPU speed can be changed from BASIC using the
CPUSPEED command to a percentage of the maximum clock frequency, which I think is currently defined to be 1296 MHz or something like that.
Changing the voltages is possible on the OPi, but that is board-specific and thus not currently supported by Engine BASIC. It might be possible to do so from BASIC, though, if you read enough datasheets...
Some details: https://linux-sunxi.org/Xunlong_Orange_Pi_PC#CPU_clock_speed_limit
(Be aware, however, that experimenting with this and getting it wrong may blow up your H3. So may getting it right and not providing adequate cooling, but that will probably take longer...)
Edit: I forgot about the DRAM clock, that of course also affects performance. Changing it is not currently supported, though. The unpleasant details can probably be found somewhere in here: https://github.com/u-boot/u-boot/tree/master/arch/arm/mach-sunxi