。
I was the slowest off the start.
Literal
I [topic-は] start [at-で] most was-delayed.
スタートで uses で for 'at the start' — a metaphorical location. 一番 ('number one, most') combined with 遅れる ('to be delayed, to be late') = 'the most delayed, the slowest.' Very natural in Japanese where English would say 'the last to start.'