OK, here are some results from one of our test rigs that runs 24/7:
As you can see there is hashrate drop with the increase of the DAG size but it is nowhere as dramatic as 50% for ETC. Additionally, with 1070Ti the drop is almost non-existing, so there is definitely something wrong with your setup. We suggest adding the -hstats 2 parameter to see the actual clocks and power consumption, the reason for power cap (pcap) if any, and so on.
Here are the result going from 310 to 311 (remove the dag % generation for readability)
Nothing stands out
C:\Users\rig01\Desktop\PhoenixMiner_4.9c_Windows>PhoenixMiner.exe -bench 310 -hstats 2
Phoenix Miner 4.9c Windows/msvc - Release build
-----------------------------------------------
CUDA version: 10.0, CUDA runtime: 8.0
No OpenCL platforms found
Available GPUs for mining:
GPU1: GeForce GTX 1070 Ti (pcie 1), CUDA cap. 6.1, 8 GB VRAM, 19 CUs
GPU2: GeForce GTX 1070 Ti (pcie 9), CUDA cap. 6.1, 8 GB VRAM, 19 CUs
GPU3: GeForce GTX 1070 Ti (pcie 17), CUDA cap. 6.1, 8 GB VRAM, 19 CUs
GPU4: GeForce GTX 1070 Ti (pcie 40), CUDA cap. 6.1, 8 GB VRAM, 19 CUs
GPU5: GeForce GTX 1070 Ti (pcie 48), CUDA cap. 6.1, 8 GB VRAM, 19 CUs
Nvidia driver version: 441.87
Benchmark mode. DAG epoch #310
GPU1: Starting up... (0)
GPU1: Generating ethash light cache for epoch #310
GPU2: Starting up... (0)
GPU3: Starting up... (0)
GPU4: Starting up... (0)
GPU5: Starting up... (0)
Light cache generated in 3.1 s (17.6 MB/s)
GPU5: Allocating DAG (3.44) GB; good for epoch up to #312
GPU4: Allocating DAG (3.44) GB; good for epoch up to #312
GPU2: Allocating DAG (3.44) GB; good for epoch up to #312
GPU5: Generating DAG for epoch #310
GPU4: Generating DAG for epoch #310
GPU3: Allocating DAG (3.44) GB; good for epoch up to #312
GPU2: Generating DAG for epoch #310
GPU3: Generating DAG for epoch #310
GPU1: Allocating DAG (3.44) GB; good for epoch up to #312
GPU1: Generating DAG for epoch #310
GPU5: DAG generated in 10.7 s (326.4 MB/s)
GPU4: DAG generated in 10.8 s (325.5 MB/s)
GPU5: Using v3 Ethash CUDA kernels (GeForce GTX 1070 Ti)
GPU4: Using v3 Ethash CUDA kernels (GeForce GTX 1070 Ti)
GPU3: DAG generated in 10.6 s (331.2 MB/s)
GPU2: DAG generated in 10.7 s (327.1 MB/s)
GPU1: DAG generated in 11.1 s (315.5 MB/s)
GPU2: Using v3 Ethash CUDA kernels (GeForce GTX 1070 Ti)
GPU3: Using v3 Ethash CUDA kernels (GeForce GTX 1070 Ti)
GPU1: Using v3 Ethash CUDA kernels (GeForce GTX 1070 Ti)
GPU1: 53C 29% 34W, GPU2: 54C 34% 34W, GPU3: 54C 31% 34W, GPU4: 52C 29% 34W, GPU5: 55C 33% 35W
GPU1: cclock 1341 MHz, cvddc 700 mV, mclock 4608 MHz, p-state P0, pcap no
GPU2: cclock 1316 MHz, cvddc 700 mV, mclock 4608 MHz, p-state P0, pcap no
GPU3: cclock 1379 MHz, cvddc 700 mV, mclock 4608 MHz, p-state P0, pcap no
GPU4: cclock 1354 MHz, cvddc 700 mV, mclock 4608 MHz, p-state P0, pcap no
GPU5: cclock 1404 MHz, cvddc 700 mV, mclock 4608 MHz, p-state P0, pcap no
GPUs power: 170.9 W
Eth speed: 152.868 MH/s, time: 0:00
GPUs: 1: 30.347 MH/s 2: 29.297 MH/s 3: 30.864 MH/s 4: 30.790 MH/s 5: 31.569 MH/s
GPU1: 56C 29% 87W, GPU2: 57C 33% 85W, GPU3: 56C 31% 87W, GPU4: 55C 29% 87W, GPU5: 57C 33% 90W
GPU1: cclock 1341 MHz, cvddc 700 mV, mclock 4404 MHz, p-state P2, pcap no
GPU2: cclock 1316 MHz, cvddc 700 mV, mclock 4404 MHz, p-state P2, pcap no
GPU3: cclock 1379 MHz, cvddc 700 mV, mclock 4404 MHz, p-state P2, pcap no
GPU4: cclock 1354 MHz, cvddc 700 mV, mclock 4404 MHz, p-state P2, pcap no
GPU5: cclock 1404 MHz, cvddc 700 mV, mclock 4404 MHz, p-state P2, pcap no
GPUs power: 436.8 W
Eth speed: 152.893 MH/s, time: 0:00
GPUs: 1: 30.368 MH/s 2: 29.288 MH/s 3: 30.865 MH/s 4: 30.802 MH/s 5: 31.570 MH/s
Quitting...
Eth speed: 152.902 MH/s, time: 0:00
GPUs: 1: 30.372 MH/s 2: 29.281 MH/s 3: 30.868 MH/s 4: 30.806 MH/s 5: 31.576 MH/s
C:\Users\rig01\Desktop\PhoenixMiner_4.9c_Windows>PhoenixMiner.exe -bench 311 -hstats 2
Phoenix Miner 4.9c Windows/msvc - Release build
-----------------------------------------------
CUDA version: 10.0, CUDA runtime: 8.0
No OpenCL platforms found
Available GPUs for mining:
GPU1: GeForce GTX 1070 Ti (pcie 1), CUDA cap. 6.1, 8 GB VRAM, 19 CUs
GPU2: GeForce GTX 1070 Ti (pcie 9), CUDA cap. 6.1, 8 GB VRAM, 19 CUs
GPU3: GeForce GTX 1070 Ti (pcie 17), CUDA cap. 6.1, 8 GB VRAM, 19 CUs
GPU4: GeForce GTX 1070 Ti (pcie 40), CUDA cap. 6.1, 8 GB VRAM, 19 CUs
GPU5: GeForce GTX 1070 Ti (pcie 48), CUDA cap. 6.1, 8 GB VRAM, 19 CUs
Nvidia driver version: 441.87
Benchmark mode. DAG epoch #311
GPU1: Starting up... (0)
GPU1: Generating ethash light cache for epoch #311
GPU2: Starting up... (0)
GPU3: Starting up... (0)
GPU4: Starting up... (0)
GPU5: Starting up... (0)
Light cache generated in 3.1 s (17.6 MB/s)
GPU5: Allocating DAG (3.45) GB; good for epoch up to #313
GPU5: Generating DAG for epoch #311
GPU1: Allocating DAG (3.45) GB; good for epoch up to #313
GPU1: Generating DAG for epoch #311
GPU4: Allocating DAG (3.45) GB; good for epoch up to #313
GPU5: DAG 14%
GPU4: Generating DAG for epoch #311
GPU3: Allocating DAG (3.45) GB; good for epoch up to #313
GPU3: Generating DAG for epoch #311
GPU2: Allocating DAG (3.45) GB; good for epoch up to #313
GPU2: Generating DAG for epoch #311
GPU5: DAG generated in 10.8 s (324.0 MB/s)
GPU5: Using v3 Ethash CUDA kernels (GeForce GTX 1070 Ti)
GPU1: DAG generated in 11.4 s (309.2 MB/s)
GPU4: DAG generated in 10.7 s (327.4 MB/s)
GPU3: DAG generated in 10.6 s (330.8 MB/s)
GPU2: DAG generated in 10.6 s (332.5 MB/s)
GPU4: Using v3 Ethash CUDA kernels (GeForce GTX 1070 Ti)
GPU1: Using v3 Ethash CUDA kernels (GeForce GTX 1070 Ti)
GPU3: Using v3 Ethash CUDA kernels (GeForce GTX 1070 Ti)
GPU2: Using v3 Ethash CUDA kernels (GeForce GTX 1070 Ti)
GPU1: 54C 28% 34W, GPU2: 55C 34% 34W, GPU3: 55C 31% 34W, GPU4: 54C 29% 34W, GPU5: 56C 34% 35W
GPU1: cclock 1341 MHz, cvddc 700 mV, mclock 4608 MHz, p-state P0, pcap no
GPU2: cclock 1316 MHz, cvddc 700 mV, mclock 4608 MHz, p-state P0, pcap no
GPU3: cclock 1379 MHz, cvddc 700 mV, mclock 4608 MHz, p-state P0, pcap no
GPU4: cclock 1354 MHz, cvddc 700 mV, mclock 4608 MHz, p-state P0, pcap no
GPU5: cclock 1404 MHz, cvddc 700 mV, mclock 4608 MHz, p-state P0, pcap no
GPUs power: 172.0 W
GPU1: 55C 29% 67W, GPU2: 56C 33% 65W, GPU3: 56C 31% 66W, GPU4: 55C 29% 67W, GPU5: 57C 34% 69W
GPU1: cclock 1341 MHz, cvddc 700 mV, mclock 4404 MHz, p-state P2, pcap no
GPU2: cclock 1316 MHz, cvddc 700 mV, mclock 4404 MHz, p-state P2, pcap no
GPU3: cclock 1379 MHz, cvddc 700 mV, mclock 4404 MHz, p-state P2, pcap no
GPU4: cclock 1354 MHz, cvddc 700 mV, mclock 4404 MHz, p-state P2, pcap no
GPU5: cclock 1404 MHz, cvddc 700 mV, mclock 4404 MHz, p-state P2, pcap no
GPUs power: 333.8 W
Eth speed: 80.350 MH/s, time: 0:00
GPUs: 1: 16.163 MH/s 2: 15.189 MH/s 3: 16.098 MH/s 4: 16.360 MH/s 5: 16.539 MH/s
Eth speed: 80.376 MH/s, time: 0:00
GPUs: 1: 16.170 MH/s 2: 15.188 MH/s 3: 16.099 MH/s 4: 16.370 MH/s 5: 16.549 MH/s
Eth speed: 80.386 MH/s, time: 0:00
GPUs: 1: 16.177 MH/s 2: 15.189 MH/s 3: 16.100 MH/s 4: 16.372 MH/s 5: 16.547 MH/s
Eth speed: 80.389 MH/s, time: 0:00
GPUs: 1: 16.180 MH/s 2: 15.185 MH/s 3: 16.100 MH/s 4: 16.374 MH/s 5: 16.550 MH/s
Quitting...
Eth speed: 80.390 MH/s, time: 0:00
GPUs: 1: 16.176 MH/s 2: 15.184 MH/s 3: 16.098 MH/s 4: 16.378 MH/s 5: 16.554 MH/s