There's no a single simple answer, except: don't decide about the implementation first, instead competently and without prejudice evaluate your actual possibilities.
Also don't decide the language like "Python" or "Ruby" first. If you expect that the calculations are going to take a month, then code them in Ruby and wait the month for the result, you can miss the fact that you could have had the results in one day by just using another language, most probably without sacrifying the readability of the code much. Only if the task is really CPU-bound, of course.
On another side, if you have a ready solution in Python, and you'd need a month to develop the solution for other language, the first thing you have to consider is how often you plan to repeat the calculations afterwards. Etc.
There is a lot to be said for confirming that you have the right logic and then looking into something like PyPy/Cython/etc. or porting to a lower-level language.
Which language do you suggest which matches this?
I don't know many people who use the language "numeric error handling characteristic" but I know the benefit of already having good written high-level primitives (or libraries).
The other area I've seen this a lot is caused by the way floating point math works not matching non-specialists’ understanding – unexpectedly-failing equality checks, precision loss making the results dependent on the order of operations, etc. That's a harder problem: switching to a Decimal type can avoid the problem at the expense of performance but otherwise it mostly comes down to diligent testing and, hopefully, some sort of linting tool for the language you're using.
In a stateless microservice architecture, disk I/O is only an issue on your database servers. Which is why database servers are often still run on bare metal as it gives you better control over your disk I/O - which you can usually saturate anyway on a database server.
In most advanced organizations, those database servers are often managed by a specialized team. Application servers are CPU/memory bound and can be located pretty much anywhere and managed with a DevOps model. DBAs have to worry about many more things, and there is a deeper reliance on hardware as well. And it doesn't matter which database you use; NoSQL is equally as finicky as a few of my developers recently learned when they tried to deploy a large Couchbase cluster on SAN-backed virtual machines.
But that's not really what the discussion is about. In the web services case, your dataset often/generally fits in memory because your data set is tiny. You don't need large servers for that, most of the time. Even most databases people have to work with are relatively small or easily sharded.
In the context of this discussion, consider that what matters is the size of the dataset for an individual "job". If you are processing many small jobs, then the memory size to consider is the memory size of an individual job, not the total memory required for all jobs you'd like to run in parallel. In that case many small servers is often cost effective.
If you are processing large jobs, on the other hand, you should seriously consider if there are data dependencies between different parts of your problem, in which case you very easily become I/O bound.
It has two flaws: One, it has nVidia Optimus (that just suck, whoever implemented it should be shot).
Two, the I/O is not that good, even with a 7200 RPM disk, and Windows 8.1 make it much worse (windows for some reason keep running his anti-virus, superfetch, and other disk intensive stuff ALL THE TIME).
This is noticeable when playing emulated games: games that use emulator, even cartridge ones, need to read from the disc/cartridge on the real hardware, on the computer they need to read from disc quite frequently, the frequent slowndowns, specially during cutscenes or map changes is very noticeable.
The funny thing is: I had old computers, with poorer CPU (this one has a i7) that could run those games much better. It seems even hardware manufacturers forget I/O