Equipment, encumbrance, and whether you have the fast roll or not actually play no part in how well you can dodge attacks. It's all about the Agility stat, which is affected by how many points you have in ATN and ADP. Put simply, the higher your Agility, the more invincibility frames (i-frames) you will have during your rolls and backsteps.
With 22 ATN and 12 ADP you would have 94 Agility, which would give you about 10 or 11 i-frames when rolling. The level of Agility that most people feel comfortable at is 100 or 105, which give you 12 and 13 i-frames respectively. That may not sound like much more but I certainly found it made a difference when I decided to beef up my ADP.
Anyway, despite all that i-frame stuff, this game
does have some wonky hit-boxes, especially in PvE. Certain attacks from bosses appear to have invisible AoE damage that makes it look like you're being hit by nothing. Here's me dodging an attack with 100 Agility and somehow still getting clipped.
http://giant.gfycat.com/VaguePowerlessAmphiuma.webm
The best thing you can do is get your Agility up and hope that the game doesn't decide to fuck you over too badly when you dodge.