Everything is slow/hangs when I'm inputting 20 files, 8 MB each

Taverna 1 is unfortunately not very good at dealing with large data - although this might seem a strange thing to say for a 'grid' project this was never an initial priority. You have two options - first is to redesign your services such that you pass references to data held elsewhere. This will generally benefit all your users as the efficiency hit you take when running large (>1mb) files through a SOAP service is pretty severe compared to the hundred or so bytes used by a reference (i.e. a HTTP URL).

The second option is to wait for Taverna 2 but this may not solve the problem if the problem is in the service architecture. Taverna 2 is capable by design of handling gigabytes of data with ease but that won't magically make any service in the workflow capable of doing that at a single point. In general if you find yourself passing large data around through a web service interface this is a flaw in the service design.