1. Why there are so many active threads about free transaction accelerators?
Since a lot of people like taking advantage of newbies and mostly offer just abuse a free service like viabtc and antpool's accelerator and then beg for tips.
2. Do you accelerate transaction you have had no control of mining fee?
Most people just accidentally send with low fees or cheap out and then want it accelerated. I am someone who's cheaped out a couple of times for transactions (even once for a campaign payment, lol). Although I see a lot of people asking for their hitbtc withdrawal acclerated.
3. Do you do it to lower transaction cost?
Most people only do it when they send it with a low fee. Bit confused by this question.
4. Do you have a control of input private key or output private key or none?
Yep, I usually do.