It has been estimated that being dehydrated can have a negative impact on exercise performance. A study published in the Journal of Strength and Conditioning Research looks at how fluid loss amounting to 2% of your body weight can impact steady state cycling.
On separate occasions, 9 recreationally active men cycled for 40 minutes at a steady pace. During one session, they were adequately hydrated. The following day they cycled in a dehydrated state. Subjects reported greater rates of perceived exertion and lower feelings of recovery during and after the dehydrated stage. Blood levels of lactate were also greater when dehydrated.