DW and I use different bathrooms. Hers is at the end of the water supply lines (no feedback line). For two years, she has been (mildly) complaining that the hot water took too long to reach the shower. I brushed it off, until I tried it myself. First of all, the shower, while having the same shower head as mine, was much slower, less water.
I swapped shower heads, and voila, the pressure and time to get hot water was ok. Now my shower was slow. I took the shower head off, and soaked it in CLR for an hour and tried it again. No difference. Next, I examined the inside of the shower head, and could see no blockage, but the tiny filter holes were colored black instead of the silver color of the other shower head.. Strange...
Apparently the filter holes were clogged by metal particles, that were not affected by using the CLR. While the cost of a replacement head is under $10, I resolved to try to fix the problem, It was relatively simple... I just used a needle tool to pick the metal corrosion off each hole. The process took about three minutes, and the "clean" head worked perfectly.
If we had not had the comparison head, I would have certainly bought a new replacement, as the blocked head "looked" clean.