I would recommend using the NVP (Name-value pair, basically POST with data) API over the SOAP API. NVP should be significantly lighter weight than SOAP. There are a few questions already on SO that complain about SOAP. I just was trying to figure out which to use and came upon those. Hope that helps.
Also, here’s how PayPal describes the NVP API:
The PayPal Name-Value Pair API (NVP API) enables you to leverage the functionality of the PayPal API by simply sending an HTTP request to PayPal and specifying request parameters using name-value pairs. The NVP API is a lightweight alternative to the PayPal SOAP API and provides access to the same set of functionality as the SOAP API.
Emphasis my own.