As part of FX startup optimizations in 6u18 FX runtime is now imported in the "system" cache and we use special optimized classloader to load runtime from the system cache.
In b05 (6894899) this code was refactored to make sure we only allow system cache entries to be loaded to the "optimized" FX classloader. However, this codepath skips use of MemoryCache and therefore every time we need jar (e.g. to load new class) we read cache entry from the disk (disk cache but we still need to load and parse it).
For example this happens 650 times on startup of Store app.
Other problem fixed as part of this bug fix is to recover support for use of "alternative"
runtimes - this is needed for performance tracking with Soma promoted builds.
Perf tests shows that with fix startup performance for minimal fx applet improves by 10+% (this will likely return us to b04 level only).