Ive been a Mercury user for a few years now and during that time a lot of features have been added; however the app is now a bit too large/bloated (at an installed size of 89MB). Other browsers are now doing almost as much with one in particular weighing in at only 14MB. This means you can have more tabs open with fewer tab refreshes.
Speed wise, its quite good although not quite up to Safari as it doesnt appear to be using the new wkwebview.
Caddoc about Mercury Web Browser Pro - with powerful Ad Block extension, v9.3.0