No constraints will often be faster, but your data will not be 100% consistent.
Over time you _will_ have child child records pointing to non existing parents and the same with foreign keys. These often don't really hurt anything, until they do. Since most of the testing is with clean data, most issues due to bad data are in production and reported by end users.
I've worked successfully on systems without any real constraints and it is usually much better to start with and remove them later as needed. Adding them later is a pain as you first have to clean up the data and fix the code issues that created the bad data in the first place.
Over time you _will_ have child child records pointing to non existing parents and the same with foreign keys. These often don't really hurt anything, until they do. Since most of the testing is with clean data, most issues due to bad data are in production and reported by end users.
I've worked successfully on systems without any real constraints and it is usually much better to start with and remove them later as needed. Adding them later is a pain as you first have to clean up the data and fix the code issues that created the bad data in the first place.