Hi
Maybe you are generating a huge quantity of invoices. Brian is right. Avoid the creation of FI documents with the proper settings in VOFA and create them after.
You can check also your summarizing FI documents. SAP Note 36353 - AC interface: Summarizing FI documents. If you are creating a lot of items in tables BSID/BSAD this increase the number of updates.
If you are using SIS, avoid the syncronous update. Are you using COPA? Check Note 553626 - FAQ Note Profitability Analysis (CO-PA).
I hope this helps you
Regards
Eduardo