Does charging and CPU usage affect each other?
The CPU is boss of the power scheme and will get all the power it requests. The charger is boss of the maximum power it will output.


Only thermal conditions throttle the CPU. 
If the charger is offering more power to be delivered than the charging controller is being asked to provide to the CPU, you have excess power available. 
When the battery needs replenishment and excess power is available, the energy will be applied there in the usual non-linear LiPo charge curve off fast charge to roughly 80 percent capacity then slow charge. 


The CPU can pull from two sources and the charger can deliver to two sources. 

It’s a pretty simple systematic first glance, but the nonlinear charge and variable CPU/GPU loads make si ole linear time estimates not match reality. 

So, if your charger is not providing the power needed to keep the CPU fully powered, the battery will drain and eventually the Mac will shut off because there’s no more battery to make up the deficit in power.  And that is why the CPU can run at full speed. 

I don’t think there is any general common timing even if you restrict yourself to only USB-C charging models. But, the great news is you can just open the terminal and check the current power budget and time estimates and not rely on me or anyone else rule of thumb on the internet. 

 pmset -g sysloadlog
 pmset -g thermlog
 pmset -g pslog


Those three commands show engineering summaries and details for the overview of the system state (sysloadlog) and the thermal CPU throttling (thermlog). The last power log will answer the specifics of any charge scenario for any model Mac and the charger you have connected.