As Rich mentioned this is not a Java forum, but one thing worth trying is a larger preallocated Java heap space.
First execution might still be lame but subsequent ones should be faster if garbage collection is what hits you.
Use -Xms400m -Xmx512m as arguments to Zulu Java for a start and eventually experiment with the numbers, remember Pis only have 1GB. Putting swap on a NAS or USB stick might also help, see this post.
