incremental compilation takes longer than flat compilation

I am currently trying to use incremental compilation to speed up the testing cycles. The goal was to see if compilation will be faster when root partition changes while preserving the post-fit options for the other partitions. However, while trying incremental comp on some of the projects, I found that the compilation time actually increased quiet a bit. I am wondering if there are any steps that might be missing from my process? Or perhaps there are other methods that I can explore? Here are the steps that I took:

1. Assigned one of the hierarchal instance as design partition, and type as “default”
2. Set preservation level to “final” for the partition
3. In compiler settings, I checked the fast preserve option in incremental compile
4. Compiled as first run
5. Modified code in the root partition
6. Compiled as second run to see the effect of incremental compilation. (Here I thought compilation would be faster, since I preserved the design partition from a previous compilation, but this was not the case)

Here are the compilation times for flat, first run and second run as reference:

Flat: 39 minutes

1^st run: 1 hour and 16 minutes

2^nd run: 1 hour and 19 minutes

Thank you and have a good day.