Yeah, armored core is a good fast mech series. Actually they made a Front Mission game that's on PC that plays a whole lot like armored core. Fast pace while allowing lots of customization of your robot and doesn't just feel like COD with robots. This is probably the best fast paced robot combat game you can get on the pc.
If you want to use an emulator theres a really old playstation game called R.A.D. that lets you control giant robots. It's not fast paced at all as you controll the arms and legs individually. Think Godzilla era fighting. But it's the closest thing I can think of to sentinels.
If you have any interest at all in playing fast paced mech games online against humans you can try Hawken. Free to play. http://www.playhawken.com/
Looks great, plays fast. But honestly you won't get many options without paying.
I'd suggest giving mechwarrior a try too. It is slower, but if you get the hang of it there is much more satisfaction to fighting. There's also a free to play mech warrior online too.