We have installed FlexCel Studio 3.7 for VCL/LCL and FlexCel Studio 5.6.5 for VCL and FireMonkey.
Since 3.7 installed, everytime we recompile a project with a FlexCelPreviewer, a FlexCelReport and a XLSXAdapter, the project takes a lot of time to be compiled (Delphi seems nearliy hanged when compiling the form with theses components).
This is weird, FlexCel 5 is big (it includes a lot of stuff, even a pdf engine), but if you aren't recompiling the full FlexCel each time, it should work fast. Can you verify that the .pas units for FlexCel 5 aren't in your library path? If FlexCel 5 is recompiled each time, it will indeed take a lot of time, but this is why the installer precompiles it when you install it.
Other thing to look at might be antivirus, can you try with them turned off? Another thing is, where is FlexCel 5 installed? Is it a network share? IT should be installed locally to go fast.
The other idea you might try is to compile with runtime packages for FlexCel5 during development, in this case it will just be a dll and compile times should be 0. You can always turn off packages for the release builds. But it shouldn't take that long in any case, I am not sure on what else can be happening.