There are to very commonly used sayings. They go like this,
Hurry spoils the curry.
Excess of nothing is good.
The same is applicable when it comes to testing. it is usually seen that people want to invest very little time in testing software. People always want that the testers do their work very fast in the very limited time that is spared for them. Now that is only possible if testers were gods. Then they would raise their palm, say TATHASTU, and viola the software would become perfect.
Humans are by nature bound to forget and make mistakes even in their natural state. So when you are testing very fast, you can very well miss some points, maybe some important bugs. But, that is how the human mind works. Fast doesn't really work out well at least not always.
Sufficient time may reduce the mistakes that us humans will make, but that most definitely doesn't mean that we should test too much. With excessive testing we might end up implementing way too many validation and restrictions on the software. This may screw up user experience. So no matter how well secured and validated your software is, it will not be a success if the user doesn't return to use it.
You should neither test very fast nor should you test too much. Try and get as much knowledge of the software, its requirements, what problem is it trying to solve? Who is the target audience? Get extra pair of eyes to double-check your work and see if you missed out something. Yes, you can never make a 100% bug free software, but you can definitely make it more user-friendly. Decide how much you would test, and take your time for it!