Multiple schema or multiple databases for better performance
It's impossible to evaluate the options without knowing what the actual problem is. 

Your first port of call should be to identify the reason the servers run slowly - it may be memory or CPU, rather than disk throughput. Perfmon will be able to tell you this; you can also run Microsoft's PAL tool to get a better insight. If it's not disk throughput that's limiting you, option 2 and 3 are pointless. 

Secondly, it may well be possible to optimize the queries - this may be cheaper, quicker and less risky than changing the infrastructure. Find the query that's running slowly, work out what's constraining it, and see if you can optimize it. 

If you do find out that the queries are optimally tuned, and that it is disk throughput that's the bottleneck, I'd put each of the two back-end DBs on separate drives - it doesn't sound like they are competing against the front DBs (as they run at different times), but they might be competing against each other if both DBs are used in overnight batch jobs.