Oftentimes in a lesson a student will ask me, when playing an exercise that's rather difficult to them, "Can I slow it down?", to which I always happily reply, sure! After all, working through an exercise slowly is probably the most fundamentally important practice habit to have. What has been happening too often lately, however, is instead of playing slower (still with solid time), they go off the rails and sort of schluff through it out of time, poorly and inconsistently.
This brings me to a key aspect of practicing slowly, for any musician, but especially for drummers: No matter what the tempo is, it's vitally important to always practice with solid time. If you're going to work on something "out of time", you're likely reinforcing a habit that will make your practicing more difficult, not less.
An obvious part of this discussion is the importance of using a metronome in ones practice. The idea of practicing with solid time, however, is more nuanced than just the simple idea of using a metronome. If you always think, feel and play with awareness of the pulse, the metronome will help you develop a deeper sense of it. If you rely on the metronome to 'provide the time for you', you're then using it more as a 'crutch'. Ultimately, the time is OUR responsibility (as musicians), and not that of any outside sources.