ContentEncodingRepository.getContentEncodings(HttpServerExchange) allocates many ArrayList iterators in hot paths for handling requests. It may be worthwhile to avoid using for-each loops and instead use old fashioned for loop with List.get(int) to avoid additional allocations during request processing.
 

java.util.ArrayList$Itr
at java.util.ArrayList.iterator()
at java.util.AbstractCollection.addAll(Collection)
at io.undertow.server.handlers.encoding.ContentEncodingRepository.getContentEncodings(HttpServerExchange)
at io.undertow.server.handlers.encoding.EncodingHandler.handleRequest(HttpServerExchange)