Hi Fedor
I struggled getting to grips with the 15s on my first cycle, how many sets of 15s are you doing? I started off doing one set of 15's but then upped it to two sets.
I also arranged the order of my exercises to fatigue the muscle group's quicker and I also took smaller rest periods between sets, so for example I will start my session as follows;
Dips x 15
flat bench x 15
incline bench x 15
decline dumbell presses x 15
If I am wiped out and don't think I could do a second set of the above I will just do another set of dips or I will do two sets of as many of the above exercises as I can, this usually finishes me off. When I move to 10's and 5's I will drop the number of different exercises as fatiugue kicks in, I will usually drop the exercise that I feel is not causing me any discomfort.
I try to order my exercises based on the muscles I want to grow the most first. I am happy with my legs now but not my chest, so I start with chest exercises, then move onto my arms and finally my legs as that is the preference I have in this cycle for growth. My leg workout would look's like this and I will drop eh number of different exercises if I don't have the energy but I will always squat.
split squats' left leg x 15
Lunges left leg x 15
front squat x 15
split squat right leg x 15
Lunges right leg x 15
front squat x 15
I will only take 30 second rest between exercises on 15s, I am trying to get myself as close to failure as I can by my 15th rep. If I cannot do 15 reps for any exercise I will cluster to 20 reps for that exercise so
12 x front squats + 4 + 4 to get me to 20 for example.
I increase the weight only on the front squat for every session, the other exercises become my warm up.
To build strength quickly you could look to concentrate more effort on bodyweight exercises such as Dips, chins, rear pullups and push up's. You don't have much fat on you so you should get to 15 reps pretty qucikly. You will do even better if you can get a weight belt and start adding weight to these same exercises. I find body weight exercises hard and they definately helped me to build strength quickly.
Your diet will also dictate how quickly you develop, if you are nto getting enough calories and protein you will have a lack of energy will not be abel to keep increasign weight at each session and will not grow. You should feel your muscles after each workout as an example I have just started my second cycle of HST and front squats are a new exercise for me, I now have the DOMs from my Wednesday session and the rest of my body is feeling teh effects of tonights session and this is only my second workout for this cycle. If your body is not telling you it hs done a session you need to push yourself more and put more effort into the workout. I would also expect you to suffer a little from teh DOMs if you are just startign out with these exercises. I could barely walk after I first started squatting and my hamstrings really suffered when i started SLDLs.
Dont forget we are all different and what might work for me may not work for you. Play around a little with the volume and weight you are usign and ty to make it fun or you won't want to do it. Use this first HST cycle to learn what works for you and which is the best order for you to do the exercises to get the most out of them.
Good luck and keep posting.