Weaker than his full power at least. Cell was seen as getting desperate and committing the same mistake as Trunks, so he was reducing his real abilities. I see Cell full power like the enhanced SSJ that was useful power-wise and speed wise (SSJG2) and the 'base' Cell that fought Goku and Gohan, like a SSJ Cell... It fits at least with a grade 3 and SSJ2 like-states being shown and the visuals for when Cell went full power seems, quite interestingly, kinda similar to the visual art depicting Vegeta going grade 2. Head canon, but makes sense. Since SSJ was still better than SSJG3 balance-wise, his speed would likely be below 'base' Cell, so maybe buff Cell couldn't even defeat Goku there.