Should POTS DSL splitter slow my connection speed?

I have ATT DSL – Elite up to 1.5 Mbps. The best speed (Ookla) I can record is 0.79 Mbps and I’m using 2Wire DSL filters on my phone lines. I decided to upgrade to Uverse (DSL only) - Elite pkg. up to 6.0 Mbps. In preparation I installed a POTS DSL Splitter and ran Cat5 to my modem. My speed decreased to .68 Mbps. I then connected the Cat5 directly to the NID and my speed was 0.82 Mbps. Thoughts? Should I expect a decrease in speed with a splitter? Or, maybe I should try a different splitter? Suggestions?