I could not keep up with everyone's reading speed and fell behind them all.
Because everyone's reading speed was faster than mine, I fell behind them.
ご質問ありがとうございます。
① "I could not keep up with everyone's reading speed"=「皆の読むスピードについていけなかった」
"and fell behind them all."=「そして皆より遅れてしまった。」
② "Because everyone's reading speed was faster than mine,"=「皆の読むスピードが私のより速かったので」
" I fell behind them."=「私は彼らより遅くなった。」
ご参考に。