@Override
        public HFileBlock nextBlock() throws IOException {
          if (offset >= endOffset) {
            return null;
          }
          HFileBlock b = readBlockData(offset, length, false, false, true);
          offset += b.getOnDiskSizeWithHeader();
          length = b.getNextBlockOnDiskSize();
          HFileBlock uncompressed = b.unpack(fileContext, owner);
          if (uncompressed != b) {
            b.release(); // Need to release the compressed Block now.
          }
          blockTracker.add(uncompressed);
          return uncompressed;
        }
If reading block cost time in milliseconds more than the threshold, a warning will be logged.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlock.java