Actually before 1:50 mark (1:48-1:49), it wasnt that Kiplimo slowed down. It's the 2 bros who went VTEC and kiplimo could not match esp with this tunnel upslope.
Visually, my guesstimation is that this is not the type of gradient that here in SG we have our tunnels at, but still it is significant (esp at this late stage of race), cyclists would fully know this feeling with tunnel upslopes.
You really need POWER to maintain the speed (or most people just use momentum and then slow down when upslope).