[SVT-HEVC-Performance] Speed/BDrate are too close between M6-M7 for some 480p content.

[GuCaixuan]

[Test environment] OS： Ubuntu Ubuntu 18.04.3 LTS SVT CommitID: ffb4e8dd

[Issue Description] 480P resolution Performance have some drop under encoMode[6-9]. Below is the test data:

[Sample command Line] SvtHevcEncApp -encMode 6 -w 864 -h 480 -i InToTree_864x480_8bit_50Hz_P420.yuv -fps 50 -b hevc_M6_InToTree_864x480_8bit_50Hz_P420_Q37.bin -n 3000 -intra-period 47 -bit-depth 8 -color-format 1 -nb 3000 -ss 1 -q 37 -rc 0
SvtHevcEncApp -encMode 7 -w 864 -h 480 -i InToTree_864x480_8bit_50Hz_P420.yuv -fps 50 -b hevc_M7_InToTree_864x480_8bit_50Hz_P420_Q37.bin -n 3000 -intra-period 47 -bit-depth 8 -color-format 1 -nb 3000 -ss 1 -q 37 -rc 0

[tianjunwork]

Thanks Caixuan submitting the issue. We will investigate.

[tianjunwork]

Run the command on master and 1.4.2. InToTree clip x 3 times. Question about the issue, from the table M7 average speed is less than M6. I suppose this is what performance drop means. M6 and M7 have very close speed. The gap is really small. I think you need to run multiple time, take average to mitigate variability.