First of all your mentality for learning streams is incorrect. Speed is only half the picture. Most of it has to do with feeling the tempo of the music and synchronising your fingers to it, at least if you care about being accurate. OD10 is also too hard to begin with, because you can't feel that hit window yet. All you practice, with a harsh hit window, is stuttering your fingers back and forth trying to keep to a blue line you can't even feel. That's a bad habit. Start with OD8 until you can get 95% on all BPM's you can play, then think about going to OD9/10.
When you get tempo down, you can adapt to the BPM, so practicing ranges 15-20bpm apart is actually quite pointless, since it's all about deducing your speed from listening to the music... not about looking at the BPM and preparing your finger speed before the song even starts.
Play songs where the tempo is easy to deduce. SHK - Identity Part 4 is excellent practice for this reason. The other 150/160/170/180 BPM maps are also excellent for this reason. Something like Ai no Naniwa, however, is not, and I have no idea why it's part of the long stream practice maps. You'll probably also have a better result at first practising different stream lengths, but not deathstreams just yet.