RocksDB JNI Slow Read Performance - database

Question

I am using RocksDB JNI and I found that reads are getting exponentially slow for this program
Initially, it's in an acceptable range. But when the program runs so that the total records reach 1 million then the read time logger prints are showing around 200-300 ms. Getting still worse, as the program runs. Am I using the JNI wrong?
long n = 0;
int counter = 0;
try(
final Options options = new Options()
.setCreateIfMissing(true)
.setComparator(new Comparator(new ComparatorOptions()){
#Override
public String name() {
return "default";
}
#Override
public int compare(final Slice a, final Slice b) {
long x = ByteBuffer.wrap(a.data()).getLong();
long y = ByteBuffer.wrap(b.data()).getLong();
return (x < y) ? -1 : ((x == y) ? 0 : 1);
}
})
.setWriteBufferSize(64 * SizeUnit.MB)
.setMaxWriteBufferNumber(6)
.setMinWriteBufferNumberToMerge(2);
final RocksDB db = RocksDB.open(options, "/PathToDB/")){
boolean loop = true;
while(loop) {
if(n == Long.MAX_VALUE){
loop = false;
}
for (int j=0;j<4;j++){
try(WriteBatch writeBatch = new WriteBatch()) {
for (int i = 0; i < 200; i++) {
String urlStr = "dummy"+counter;
counter++;
Long score = getScore(urlStr);
ByteBuffer buf = ByteBuffer.allocate(Long.BYTES);
buf.putLong(score);
writeBatch.put( buf.array() , urlStr.getBytes(UTF_8));
}
long st = System.currentTimeMillis();
db.write(new WriteOptions(), writeBatch);
long et = System.currentTimeMillis();
logger.logMessage(Test.class.getName(), Level.INFO, "RocksDB write of 200 URLs successful. Time taken - {0}", new Object[]{ et-st});
} catch (RocksDBException ex) {
}
}
byte[] firstKey = null, lastKey = null;
int readC = 0;
long st = System.currentTimeMillis();
final RocksIterator it = db.newIterator();
it.seekToFirst();
while(it.isValid() && readC < 50){
lastKey = it.key();
if(firstKey == null){
firstKey = lastKey;
}
it.next();
readC++;
}
long et = System.currentTimeMillis();
logger.logMessage(Test.class.getName(), Level.INFO, "RocksDB read of 50 URLs successful. Time taken - {0}", new Object[]{ et-st});
if(lastKey != null){
db.deleteRange(firstKey, lastKey);
}
n++;
}
}catch (Exception e){
logger.logMessage(Level.SEVERE, Test.class.getName(), e);
}