[julia-users] Unexpected slowdown

[Achu]

I have a simple piece of code that finds me 10001 prime numbers. 

function a()
pl=[2]
n=3
ct=1
while(ct<10001)
    isnprime=true
    for a in pl  
        if n%a==0
            isnprime=false
            break
        end
    end
    if isnprime
        push!(pl,n)
        ct+=1
    end
    n+=2
end
    return pl
end

When I tweaked the code to check only prime factors less than the sqrt of 
the number, it slowed it down by a factor of 3.

function a()
pl=[2]
n=3
ct=1
while(ct<10001)
    isnprime=true
    for a in pl[pl.<sqrt(n)]
        if n%a==0
            isnprime=false
            break
        end
    end
    if isnprime
        push!(pl,n)
        ct+=1
    end
    n+=2
end
    return pl
end

Why is that?