Why does the encoding time increase dramatically when k increases from 34 to 35?

My setting: 64MiB chunk size. I test from 32+4 to 35+4 . CPU: Intel(R) Xeon(R) Silver 4110, Memory 256 GiB, Ubuntu 16.04. The results are as follows:
```
yaoqr@node-NF5280M5:~/Code/test$ ./encode 32 4
encode with avx2: 514.764
yaoqr@node-NF5280M5:~/Code/test$ ./encode 33 4
encode with avx2: 594.76
yaoqr@node-NF5280M5:~/Code/test$ ./encode 34 4
encode with avx2: 863.95
yaoqr@node-NF5280M5:~/Code/test$ ./encode 35 4
encode with avx2: 1583.09
```
The time is measured in ms. Anybody knows the reason why the time jumps too fast from k=34 to k=35?
Thanks!!