I tried to reproduce the issue with clocks and low speed until miner restart.
Windows 10, latest blockchain drivers, stock RX580 Nitro+.
1. No overclock: If I start miner in ETH-only mode, it shows about 16MH/s only (and low memory controller load in GPU-Z) and I see this speed for about 20 seconds, then it shows 24.6MH and works fine. For dual mode it shows full speed immediately. Unfortunately, I have no idea why GPU works slowly within first 20 seconds, it is something related to drivers.
2. I close the miner and apply some changes in clocks: memory 2000 -> 2100, core 1300 -> 1100. When I start miner, I see that it applies new clocks in about 10 seconds, but still shows 18MH/s only for first 20 seconds. Then it shows about 25.5MH.
Note that I use default ROM so these speeds are not high.
3. I close miner and apply new clocks again: -cclock 1000, -mclock 2050. Same behavior: it shows low speed at start, then normal speed for these clocks. GPU-Z shows changes in clocks in about 10 seconds after start.
So right now I cannot reproduce the issue with new clocks: in my tests new clocks are applied immediately, I don't need to restart the miner or reboot the system.
Same here, Gygabyte RX580 custom, w10, aug 11 drivers, ETH only. Everything stable. What I also noticed is that if I reboot, after the initial low hash seconds, sometimes 1 min or so, it recovers to the normal hash but the voltage at the wall is 50W lower when normal speed is restored. Then, after another minute or two, the power consumption goes up again with the same normal high hash speed.