With the changes of new tools and code in the newest version, on some computers 2018 takes more resources. I think every new build version of the app since CS6 (the first version I used) this has been the case. And it affects some users more than others, depending on their gear, media, and typical work practices and effects used.
"Slightly" older isn't how I'd describe a six year old computer used for heavy work like video editing. But then, that's only opinion ... 
The media you're using is nasty stuff for editing ... that A7Sii for example is most surely long-GOP 4k, which hammers the CPU and cores/threads/RAM for the incredible amount of de-encoding/de-compressing required per frame. If you have multiple tracks of that plus FS7 going, you're working the CPU and RAM pretty hard. Which sounds exactly like the situation you describe.
A couple options can work ... first is to transcode the A7Sii media to a digital-intermediate quality Cineform or DNxHR prior to importing, use those for editing, and dump them when the project is done to save space as you can always recreate them from the originals. You keep good quality and get a better editing process.
The next is to use the built-in Proxy media option within PrPro, and create Cineform proxies for your 4k media either on ingestion or from the Project bins themselves. Then use the + icon on the far right of the Program monitor to see the rest of the controls available, and drag the Toggle Proxies icon to your control block.
With the proxies available, click that to toggle it on (blue), and you can play back with the proxies. Toggle it off (gray) and you can see the original media.
Neil
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