Database for timeseries data
Creating 8M new records should take seconds, not hours.  You just need to do Bulk Loading instead of slow-by-slow loading.  (calling COMMIT after every row would be a "bad idea" as it will limit you to 1row/5ms).

Same concept goes for UPDATE of the data.  (Although. I would expect minutes).  Bulk operations is key.

From my experience,  your main concern will be Disk I/O performance.  You improve this by using RAID systems.  You'll probably have an extra terabyte or 2, but your performance,  and redundancy, should be greatly improved over current setup.

Which RDBMS?

I would go with Oracle.  But, that is because local resources (me) are experts in that technology.  (It also comes with a cool web based application development frame work).

Actually, I recommend you try out each of them with your actual data and pick the one that fits your company the best.