In terms of actually understanding how a hypothetical portfolio would have performed, something that is much more meaningful then the overall ROI provided by Google Finance is the concept of the annualised return.

We built our portfolio functionality with this in mind to empower investors to understand their true returns, respective of time. It often amazes users here at Australia's #1 stockmarket app, disclosure: I founded and bootstrapped the business and it's iOS/Android only at the moment.

Refer: https://stocklight.com/faq