The CPU is definitively faster. Skylake has improved branch predictors over Haswell, more execution units, larger scheduler etc. etc. And it definitely performs better on numerical code.
I can imagine though that a compiler might not benefit from these things. If the compilation process involves a lot of cache misses and unpredictable jumps, the architectural improvements of Skylake might play a very minor role. And of course, there are things such as filesystem latency and I/O latency in general...
It has better capabilities due to newer technologies but in terms of faster, it's very subjective unless you're running the processor for something it's "good" at. For general most users and developer tasks it's not that much better or not good enough to warrant an must upgrade.
Skylake is more efficient then the older Haswell especially with power usage but the Haswell is still more powerful and faster on some tasks despite being a processor from 2012. And here, in Xcode, it seems that Haswell is still king vs the Skylake equivalent.
I have a Dell 5510 with the Xeon Mobile E3-1505M processor running at 2.8GHz and 3.7GHz turbo. On paper, it has a larger L3 cache (8MB), nearly the same TDP (45W) and better turbo boost, speed step, etc. compared to my older Late 2013 MacBook Pro running the i7-4960HQ processor that runs at 2.6GHz and 3.6GHz turbo. The L3 cache is smaller (6MB) and runs nearly the same TDP at 47W. Comparing it here, it would seem that the MBP running the Haswell processor would be:
1) Slower, due to lower clock speed.
2) Hotter, due to the higher TDP
However, just a blank test in benchmarks, it's a complete opposite.
* These processors are both Quad core w/ Hyperthreading.
Running Geekbench, at the same time after a clean start up, let the system sit for 5 minutes to let all background processes settle and a clean install w/ all updates and running the latest drivers and OSes produced these results in Geekbench 4.10.
Single Core:
E3-1505M @ 2.8GHz - 4230
i7-4960HQ @ 2.6GHz - 4401
Multi-Core:
E3-1505M @ 2.8GHz - 14721
i7-4960HQ @ 2.6GHz - 15337
That's amazingly different. Now in handbrake, converting a 1080p MKV video to MP4 with the same present also showed a difference with Haswell being king. Keep in mind, there is a possibility that OpenCL played a role; I'm not sure.
E3-1505M @ 2.8GHz - 28:11 Minutes
i7-4960HQ @ 2.6GHz - 27:54 Minutes
Now let's take into account possible throttling due to heat. Nope, no throttling. The CPUs were pinned at it's max turbo speed for all 4 cores the entire time. In fact, the Dell ran vastly cooler then the MBP and the MBP still was able to hold its own even after multiple runs.
Dell CPU max temperature: 79 C / Range observed: 51 - 79 C
MBP CPU max temperature: 98 C / Range observed: 66 - 98 C
The difference isn't big and on a daily thing; I see no difference in speed. I do a difference in power usage though. The Skylake processor seems to be more using way less power then the older Haswell. Overall, if you only care about raw performance; you'll be disappointed due to the fact that it's not that much faster or if at all faster on some tasks. There are some programs and processes that Skylake will beat Haswell out of the water. However, for most users; it's not big enough to warrant a "must" switch.
And for kicks, Apple really screwed up on the nVidia drivers in Sierra. In Civ6, it is faster to use the integrated video card then it is for the nVidia. Running the same settings and resolution:
Intel Iris Pro 5200 - 34FPS average benchmark
nVidia Geforce 750M - 16FPS average benchmark
nVidia GeForce 750M (with nVidia macOS drivers) - 17FPS average benchmark