Has compilation of big programs become excessively sluggish in v 10.4?

That 3 seconds in 10.4 is an improvement over 5.3 s in 10.3. :)

I can't imagine any scenario where how you construct the function affects its formatting (assuming you're creating the same). I guess the fact that it isn't helped by disabling iconic elided forms suggests the issue is elsewhere. Also, does disabling the outuput (using a semi-colon) still result in a slowdown? That means the slowdown is either in evaluation or in the transmission over MathLink to the kernel. If you have a concrete example, that would be helpful.

The new iconic forms were made interpretable, meaning if you copy an output into an input cell (or hit Cmd/Ctrl-Shift-L), it will parse and evaluate correctly. This means the formatted forms are much larger in size, since they contain complete info for reconstructing the CompiledFunction, and hence notebooks containing them in output will be much larger. So you have a lot of these in output cells, you'll have much bigger notebooks. You might want to disable the icon forms or suppress output to save space (and the time saving the notebook) in addition to the speed up from not formatting them.

As of version 11, the bug has been fixed. Compile now performs like it did in version 10.3 and earlier.