2 sentences

1. 我跟不上他的速度。

   wǒ gēn​bu​shàng tā de sù​dù.

   I can't keep up with his speed.

   Source: Dong Chinese Dictionary

2. 他反应迟钝，总是跟不上大家的思路。

   tā fǎn​yìng chí​dùn, zǒng​shì gēn​bu​shàng dà​jiā de sī​lù.

   He is slow-witted and always fails to keep up with the group's thinking.

   Source: Dong Chinese Dictionary