The investment in speed and other user benefits was a loss-leader for the goal of increased control. The speed and other benefits were indeed a net positive for users (and motivated Mozilla to focus on performance in Firefox) but they were investment made for payoff.

You've said it yourself: "the big selling point was being faster."

And yes, there is obviously some marginal value for Google in simply making web experience as fast as possible because it keeps people using the web. But that extended web use is of no good to them if users are doing so at non-Google properties that don't see Google ads, so it seems implausible that control was not a key motivator.