Compiler defaults are to compile with maximum optimization (-O3). The shorter compilation time with newer compilers is probably unrelated. Because there have been reports of excessive compilation times, some improvements have been made in recent versions that are aimed at reducing compilation times.
Compilers include complex sequences of transformational phases, many of which are driven by tunable heuristics. Interactions with complex pieces of code are hard to predict and so regressions on some kernels invariably result when changes to the tool chain are made, with the overall distribution of speedups approximating something like a (possibly skewed) normal distribution.
I encourage programmers to file bugs for any significant regressions. To allow debugging, such bug reports must be accompanied by code that reproduces the issue. Data in bug reports is visible to the filer and NVIDIA engineers. You can try to simplify and/or obfuscate your code if necessary.