There does reach a point where playing slowly and keeping in time is harder than playing fast and keeping in time.
That said - if you're having trouble keeping the beat at 60bpm, which is about the tempo for a slow blues, practice playing just a single note in eighth notes or triplets until you can keep the time. Yes it will feel like you are being held back - that's kind of the point. This gives you plenty of time to make you chord changes, strum accurately, etc... Then, when you gradually speed it up, everything will stay nice and tight.