Q:

As a new technical lead, how can I evaluate junior (software) engineers fairly? 

I have been the technical lead in a group of 8 software junior engineers for the past 4 months.  This is my first technical lead position and it comes with the uneasy responsibility of assessing each member's performance. 
I am facing a personal mentality issue and I wish to make sure to resolve this issue moving forward so that it does not affect the fairness of my evaluation:

I have a senior role in the company due to being a top 1% performer, but in terms of years of experience in the field, I am only 0-1 years senior to all my mentee.  As such, I find it difficult to avoid comparing my mentee's performances to my own when gauging their productivity. 

This mentality leads to frustrations at times.  For example, on the extreme, 2 junior members of the team would consistently take up to 50 times more time than I would in deriving solutions to more complex tasks, even with assistance.  I can't help but to wonder, "how slow is too slow?"  in this case.  Is it fair to hold team members to the same standards I hold myself to or something that I should avoid all together as a technical lead? 
What would be a more objective approach moving forward?  

A:

Don't overthink this. 
You have 8, so compare them to each other to find a middle ground, then work from there. 
The middle ground may low or may be high, that is for you to work out.  I usually take the worst and class him/her as mediocre and the best as good unless there is something exceptional about one of them.  Then work my way up from the bottom trying to think of positive things about each as I work.  
Coding skills is not the total of what makes a good worker, manners, ability to teamwork and a bunch of other things need to be factored in.  These you assess individually and they're important.