That's too bad--HTC tends to lock their bootloaders, which is a pain.
Getting back to your original question, here are some games I like (not all of them free) that should work pretty well on low-spec devices:
Doom & Destiny: retro, funny, and fun RPG. I got it free when it was the Free App of the Day on Amazon.
Gunman Clive: fun side-shooter, quirky graphics
Jones on Fire: 8-bit retro fun
League of Evil: more 8-bit fun
Meteor Blitz: nice take on Asteroids
PewPew: lots of fun, retro vector graphics (can you sense a theme here?)
Pinball Ride: probably the best pinball game for phones, since it doesn't try to cram a ton of detail in
Plasma Sky: space shooter, lots of fun
Pudding Monsters: I enjoy this more than Cut the Rope.
Quell: very Zen
Sinister Planet: nice take on Sinistar
Gyro: warning, can be very addictive.
Grabatron: alien invasion from the alien's perspective--might not run smoothly.
Major Mayhem: also might not run smoothly, but worth a try.
Hope that's a good start!