Fast Lane - doesn't make sense

Again it depends. here's why.

2.4ghz is able to broadcast further and through more materials than 5ghz. But it is significantly slower and more sensitive to interference. 

5ghz is much higher throughput but can't broadcast as far or through as much materials.

so in certain instances going router------2.4ghz-----extender-----5ghz-----devices      might be faster because the extender is further away or has more materials blocking between router---extender

In other instances, some prefer to go  router-------5ghz-----extender-------2.4ghz-------devices.   this might work better if for instances you've got a computer hardwired into the extender and want the faster connection to the router and are close enough for a good connection. And/or need the 2.4ghz to reach devices futher away. Take for instance putting a extender at the edge of your home but wanting to reach something outdoors (challenging becaue exteriors walls block more).  

That's why its always not a bad idea to try both modes. 

As well as try moving the extender around. Sometimes things in the walls (hvac for instance) can be blocking and a lateral move can actually help more than moving closer to the router. Sometimes even moving further can help if it removes that obstruction. 

Another thing is because 2.4ghz has less available channels and the extender running off the same channel the router does, trying different channels can reduce interference and let the 2.4ghz operate more effeciently. 

So there's lots of optimizing that can be done with extenders.