Civilization is just very CPU demanding, it's the way it is.
It's a game that also tends to gobble up any CPU cores you can throw at it, so more the merrier. - I wouldn't personally be playing it on anything less than a 6-core CPU with HT.
What hardware are you running?
AMD FX(tm)-8320 Eight-Core Processor
AMD Radeon HD 7700 Series
RAM 8 GB of Something or another.
A small solid state for booting windows.
A 500 GB regular HDD of some sort.
Some sort of MSI board. I forget. A friend helped me pick the board years ago.
The AMD FX is your issue there.
Even when released it wasn't exactly the fastest thing on the market.
Your GPU could also do with a boost as well, the Radeon 7700 card despite being fantastic value back in 2012, hasn't aged well.
In short... Civilization 5 is a better fit for your PC, even then I would personally still upgrade.
Maybe "for a game in 2018" you shouldn't use a CPU from 2012. To add to it AMD has a notoriously low single thread performance. Civ hasn't gotten less complex over the years.
Well. The single threaded performance isn't really the issue for Civilization...
It's that the multi-threaded performance of the FX chips, tend to not be great anyway.
The turn calculation is single threaded since you really can't parallellise it. Any additional cores are just used to run UI.
From my extensive testing, there were gains when I went from 2-12 threads with Civilization 5.
I would imagine Civilization 6 has taken it farther. (But can't verify as I haven't played/tested it.)
With that in mind... A game doesn't actually need to leverage every CPU core for you to see a gain with more CPU cores.
I.E. Games that use 4 cores still saw improvements on my old 6-core processor.
In the AMD FX's case however... It is sharing an FP unit between two threads, hence, doubling the cores needed is extremely beneficial.