https://github.com/apache/dubbo/blob/501e4dbdf9d93b8d5b59400cff3bdac5ea89fbc4/dubbo-rpc/dubbo-rpc-api/src/main/java/org/apache/dubbo/rpc/protocol/AsyncToSyncInvoker.java#L54

The above `result.get()` will eventually delegate to [CompletableFuture.get()](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletableFuture.html#get--). I checked the implementation provided in JDK 8, it will first spin-waiting for the countdown of 256 (the eighth power of 2) and then park the thread. I believe it's the unnecessary spin that caused the bad performance.

When running under JDK 11, the performance performs well as the previous versions, I still does not check the difference between these two implementations yet.

Thank @Jeff-Lv for help finding this issue.
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package org.apache.dubbo.rpc.protocol;
import org.apache.dubbo.common.URL;