I find the same thing.
BTW, there is no square root of 12 modulo 17 :
sage: R1.<t>=Zmod(17) sage: %time (t^2-12).roots() CPU times: user 396 µs, sys: 2 µs, total: 398 µs Wall time: 407 µs 
Direct computation seems faster in this case, either in
sage: %time [t for t in Zmod(17) if t^2==R1(12)] CPU times: user 188 µs, sys: 0 ns, total: 188 µs Wall time: 194 µs 
and even faster in
sage: %time [u for u in range(17) if (u^2)%17==12] CPU times: user 87 µs, sys: 0 ns, total: 87 µs Wall time: 93.5 µs