Why wasn't Future Trunks able to go SS as quick as Kid Trunks?

[Smilodon]

I think it's because future Vegeta was weaker than present vegeta when they had sex to have kid trunks.

[Xyex]

My in universe explanation has always been that Vegeta didn't reach SSJ until after Trunks was concieved in the future, but achieved it before his conception in the past/present timeline. Vegeta being able to transform switched the transformation gene into the 'on' position in his DNA, so it was already active in Trunks from the time he was born. Also why Goten could transform so easily, Goku had already done it by then as well.