2 sentences

1. 他的进步可谓神速。

   tā de jìn​bù kě​wèi shén​sù.

   His progress can be described as rapid.

   Source: Dong Chinese Dictionary

2. 他的进步神速，短时间内就掌握了这门语言。

   tā de jìn​bù shén​sù, duǎn shí​jiān nèi jiù zhǎng​wò le zhè mén yǔ​yán.

   He made rapid progress and mastered the language in a short time.

   Source: Dong Chinese Dictionary