To me, performance is a very subjective thing. A not so fussy boss may say OK while a fussy boss may say he is no good. It is not clear-cut, unlike poor punctuality, constant absence which are misconduct that can be inquired into under Section 14 of the Employment Act by way of domestic inquiry. If you think the employee is really below expectation, then he has to go. You will give him an normal termination of service by giving him the necessary notice and other payments due to him.
For notice period, you may refer to Section 12 of the Employment Act :
(a) four weeks' notice if the employee has been so employed for less than two years on the date on which the notice is given;
(b) six weeks' notice if he has been so employed for two years or more but less than five years on such date;
(c ) eight weeks' notice if the has been so employed for five years or more on such date; "