You can achieve this using DSO for example to only copy the data that are relevant for archive or reporting if you have two ARS environments, you should check documentation "BMC Remedy Distributed Server Option Guide" (7.6.04 for example) to see if it matches your needs.
AR provides archiving functionality where you can archieve data from regular forms to another regular forms.
To improve db performance, you can write a script( e.g Shell Script if your AR server is hosted on LINUX) to read this data from archived form and push it to a a excel spreadsheet over a network.
This ways you can keep track of old data for any AR regular form.
Hmmm what i see over here is when we talk about ITSM, there are main forms like HPD:HelpDesk, PBM:Problem Investigation and CHG:Infrastructure Change.
Mostly the operations involves creating/searching/updating the transactional records on these base forms, so if our aim is to improve the performance on these forms to allow the users to perform Submit, Search and Modify seamlessly, then we can Archive(using Form Properties there is an inbuilt Archive Mechanism available(to archive and delete it from the source) and the data in these Forms would be backed up in a Archive Form(there will be a each Archive form gets created in the backend for each of these base forms when Archive is triggerd).
This would be your first step and so on you can find out the dependency forms of these base forms and trigger Archive over there as well...
Can i have to archive all the related forms which contain data related to Incident ar some specific forms.