Encoder performance

I’m using https://github.com/NVIDIA/video-sdk-samples to capture screen,but I find encoder time is huge difference with the same picture(https://www.testufo.com/frameskipping).I use 144hz monitor,the dxgi will capture screen every 6.9ms.
the gpu view are below

so what’s the problem?
my encoder parameters in program:
GPU in use: GeForce GTX 1070
[INFO ][11:27:49] Encoding Parameters:
codec : hevc
preset : p1
tuningInfo : ultralowlatency
profile : (default)
chroma : yuv420
bitdepth : 8
rc : vbr
fps : 30/1
gop : INF
bf : 0
multipass : 1
size : 2560x1440
bitrate : 30000000
maxbitrate : 60000000
vbvbufsize : 0
vbvinit : 0
aq : disabled
temporalaq : disabled
lookahead : disabled
cq : 0
qmin : P,B,I=0,0,0
qmax : P,B,I=0,0,0
initqp : P,B,I=0,0,0
nTotal=2000, time=6.35434 seconds, FPS=314.745