AppVeyor builds all jobs serially so it's important not to have too many of them to avoid builds taking too much time and queuing up.
I suggest to remove the following jobs:
- the Release build with Ninja and VS2015; we already have both a Release build with Ninja and VS2017, and a Debug build with Ninja and VS2015
- the two NMake builds: we already exercise the Ninja (cross-platform, fastest) and Visual Studio (standard under Windows) build chains
Max Risuhin you added some of those jobs, do you have any concerns?