How Crippling is -SPD +RES Tana?

Well ... she still have a really nice speed but i doubt she will double any strong units.
If you really want you can build her into a Hybrid Wall putting something like Distance Counter, Quick Riposte (or something else) and Distant Defence for Seal slot, she will counter many mages and physical units. But i think it will work better if you put the Summoner Support in her as well.

I have the same IVs, and I don't use her much, but she's good as a Guidance Anchor for mix teams.