[rbanffy]

> they have no real advantage in cpu speed

A Z15 core runs at 5.2 GHz (IIRC) and has shared access to 960 MB of L4 cache for every group of 4 sockets of 10 cores each (Linux workloads can do SMT2 on each core). They emphasize single-thread speed because they measured diminishing returns when adding more cores to an LPAR and figured out it was pointless to play a numbers game.

> These days if you can parallelise those calculations

Yes, but it's not all workloads that are amenable to that - some will want to keep a consistent in-memory representation of the working data with all cores working on the same data. If you can scale out, great. If you can't, this is the very top of the line. It you need to scale up from a z15, I suggest you wait for the z16 availability ;-)