Hashrate
Calculated hashrate based on sent shares during last 30 minutes. After mining start first 15 minutes speed will be shown as "calc...", don't worry if you see shares in column "Last share"
If your last share older then 5 minutes, "last share" time will be red.
Low hashrateIf your calculated speed below 60% threshold over a long period of time, you send low amount of shares.
It's because of bug in some versions of ethminers. Just restart miner. Also don't use buggy Genoil v1.4
In stats and API such miner marked as "BELOW_THRESHOLD" (based on last hour calculation). If you solved the issue, you can still see such message next hour.
That your mined was healed, check amount of shares last and current hours as well shares chart
Such warning possible only for miners that send eth_submitHashrate parameter. Also you can check and set variable GPU_MAX_ALLOC_PERCENT to 95 or 100%