When is the startup time more than 100 seconds? You can try something really experimental like graalvm native compilation that should reduce the startup time massively.
Instead of investing too much time on other issues: can you just try to use a non-VPS system? (Again we have seen slower response times for VPS but only in the 20% range)
UseG1GC
Unlikely that this improves something as jdk 11 has this as default option now.
Other GCs you can try are shenandoah GC or for linux systems ZGC.