For Oracle/OpenJDK 8 the default collector on most machines is the Parallel Throughput Collector,
except for some 32bit windows machines where it can be the Serial GC.
Xincgc is CMS in incremental mode. The main benefit you're seeing probably is caused by switching from the Throughput Collector to CMS, not from the incremental mode, which is designed for single-core CPUs.
Incremental Mode is also deprecated, so simply enable CMS via -XX:+UseConcMarkSweepGC
and see if that works for you.
Of course you can also try G1GC, which is also designed to reach low pause time goals and has the advantage that it does not suffer from fragmentation like CMS does and thus is less likely to experience concurrent mode failures which result in a single-threaded stop the world collection.
So, try both and measure.
See also: Oracle's Java 8 GC Tuning Guides
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…