It's not only that. It's also the fact that Goku was confident enough in fighting Pure Boo without resorting to gathering all of his ki, and was annoyed when Boo's ki hadn't decreased.
Chapter: 510 (DBZ 316), P12.1
Context: after Goku fights pure Boo for awhile
Goku: “…Dammit…! Like I thought, even if I keep doing the same thing, that bastard’s stamina doesn’t fall…! He just returns to being good as new right away…”
Chapter: 510 (DBZ 316), P13.1-6
Vegeta: “Don’t hesitate for my sake, and finish him off! With that Super Saiyan 3, you should be able to completely wipe out Boo with your ki once you gather it with all your might…!”
Goku: “Ye…yeah…I’ve been thinking of doing that as well since awhile back, but…I haven’t gotten the chance.”
Vegeta: “Eh?”
Goku: “If I want to wipe him out, I gotta gather ki for about one minute.”
Vegeta: “One minute?!”
Goku: “Dammnit~~~If it was with the Potara, I could have done it in one blast. …..Cheh~~~I guess I went and showed off too much. But I thought things would go a little better than this…!”
Vegeta: “…S …So you weren’t thinking of me…”