Firing an employee is an unpleasant experience, but it is often a necessary evil for any entrepreneur or business owner.
To be blunt, if someone isn't very good at their job, or they do not fit in with a company's culture, then they are probably not going to enjoy the role. An unhappy employee never results in productive work in the long run.
It can also reflect badly on your managerial skills. By not firing an under performer it can make you look like you don't care about the success and direction of the company. Also, it means that people working with that employee have to pick up extra work, further lowering morale and productivity.
Being decisive is an important skill for any entrepreneur, particularly in a fast-moving market. Making a quick decision to dismiss a staff member means that you can get on with finding someone more appropriate who will enjoy the role and take the company to the next stage of growth. Although there may be initial recruitment costs, dismissing the underachievers will pay off in the long term.
If you are able to realise that the person is not up to scratch within their probation period then that should make the process easier (after all, that is the very purpose of a probation period). Make sure that you actively assess someone's performance because it will also provide you with evidence to justify the dismissal.