I am running the magento 2.2.3 version on AWS EC2 currently on c5x.large the performance seems fine for me with proper page speed backed by cloudfront and Redis.
Due to cost optimization I have decided to use m4.large instance and saw the degradation in performance magento page speed from 2.5 second to 6.6 second. I noticed the cpu usage with m4 large type instance it was going up during cache creation and was neutral at other time. I also noticed the cache flush operation from magento admin panel which took approx 3.5 min where as in case of C5 xlarge it was taking 50 seconds to complete same operation.
Is something wrong with my application or it's the cache operation that has direct connection with my cpu? What will be the right Instance series to be choosen for magento 2.2.3 on production?
Also this was not the case with magento 2.1.6, the cache flush operations are very normal even with t2.medium instances we used t2.medium for dev instances earlier.
You are not only halving the CPU but also moving from a faster CPU to a slower CPU and a faster hypervisor + hardware to a slower one.
Magento generally performs better when you allocate more CPU (and faster CPU) as well as Memory to it.
If you are running a live eCommerce store and conversion is important to you (which it should), I wouldn't go with a weaker hardware and slower performance just to save a small amount of cost as it will cost you a lot more in the long run.