I guess you could consider 0.5 cm in 4 weeks slow, but then again, you could consider it a decent gain. I haven't seen a true gain in over 4 weeks, but oddly enough when I look at myself, I just seem bigger. Guess that's a good mindset to have, even if your mind is being fooled somehow.
Anyways, my point is that any gain is better than no gain. And if you aren't gaining at a rate that you like, maybe you should try different techniques than you have been using. There are numerous Penis Enlargement exercises to try, and it seems that you just have to try a lot and see what works for you. And if something isn't working anymore, maybe try something else out. I kind of think it's like weight lifting; if you keep doing the SAME EXACT routine for months on end, you will for the most part stop gaining muscle mass. This is because you body gets used to what you are doing and is anticipating everything you are doing, and compensates as such. Sometimes you have to just try new exercises in a different order with a different intensity to get the results you want. Just need to shock the body every once in awhile to get it to respond in the way you desire.
Now, I'm not saying that changing up your routine will magically give you two inches of length, but it may be what you need to increase the speed at which gains are occurring. Just keep a positive mindset, and stay persistent...the gains will come.
On that note, I think it's about time I change up my routine a bit. Hope what I said helps out.
- d_s