Use empty layouts (nothing on them), just to establish context;
Use ExecuteSQL instead of summaries (worth trying)
Calculate things in a script performed on server which returns results via Exit script [result]
Thanks for the suggestions. Never knew I could use an empty layout - does this mean that, even though I may be pulling data from the fields, they don't need to be there?? Would this be the same for the Set Field side of things as well?
I'll look at the SQL and PSOS side of things as well.
Yes, fields do not need to be on the layout in order to read from them or write to them.
To avoid jumping to and from the different layouts 15 times, you can also make use of repeating variables. So you can collect data from all 15 records, then change only layout only once.
They only need to be there if you use script steps like copy/paste or whose name begins with "Insert", like Insert Calculated result.
Thanks for the help here everybody. Going to look at everything mentioned but will take a little time to fully work through!