Why does this take such a long time?

sage: M = random_matrix(ZZ,2000,1000)
sage: T = M.change_ring(GF(2)).left_kernel().basis()
sage: %time r = T[0].change_ring(QQ)
CPU times: user 73.49 s, sys: 0.33 s, total: 73.83 s
Wall time: 74.01 s

Especially compared to
sage: %time r2 = Matrix(T).change_ring(QQ).rows()[0]
CPU times: user 7.40 s, sys: 0.01 s, total: 7.41 s
Wall time: 7.40 s

Or even
sage: %time r3 = vector(QQ, T[0].list())
CPU times: user 0.01 s, sys: 0.00 s, total: 0.01 s
Wall time: 0.01 s

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