Some of them know, couple of them quite new hires who started with a very bad performance (I've no clue how the hell this is possible! - maybe my fault)

Also I openly discussed all these issues and invite them to find a solution all together. So far not going well.

In some cases after being informed of possible serious consequences their performance got back to normal for a month or two then cycle repeated itself. So maybe every 2 months I should remind them that they very close to getting sacked.