It's a server side problem, the update of all the information tidyplates requires are huge and badly handled by the wow core slowing down the creation of the nameplates.
You should try by enabling the caching, it should speed up the whole addon.
To be honest there'arent decent alteratives out there at the moment, two addons that come to my mind are:
DocsNameplates (still working but it hasn't been updated since january).
or
NamePlateAdvance (sometime has got some little bug that requires you to press v to refresh the bars just like the tidyplates error).
My suggestion is, just stick toTidyPlates.