Slow construction

1. Construction of this ComponentArray is very slow
   n=3000; a=rand(n,n);b=rand(n)
   @btime C=ComponentArray(a=a,b=b);
   This takes more than 9 seconds even after the timing stablizes. Meanwhile, the following will be very fast
   @time C=ComponentArray(b=b); C=ComponentArray(C,b=b,a=a) #--> 0.002 seconds
   If you switch the above around it will be slow
   @time C=ComponentArray(a=a); C=ComponentArray(C,a=a,b=b) #--> 11 seconds

2. It will also be nice to have a constructor in this format
   n = 3000;
   symbol_list = [:a, :b, :c]; size_list = [(n,n),(n,),(n,n)]
   C=ComponentArray(data, symbol_list, size_list); # Data may be pre-allocated, or it could be omitted, in which case the code calculates how much total is needed from size_list.
   Or it could be like this:
   ax=ConstructAxis(symbol_list, size_list);
   C=ComponentArray(data, ax)

Start by reproducing the two ComponentArray construction timings in the issue with n=3000, then trace the constructor path used for multiple named components. Compare the fast incremental construction with the slow combined construction; done means the reported slowdown is addressed and the requested constructor form is clarified or implemented.