Efficient universal problem solver?
Levin's universal algorithm is an algorithm such that $t(U,A) &lt; s_V t(V,A) + t_V$. By modifying the algorithm (see for example Hutter's The Fastest and Shortest Algorithm for All Well-Defined Problems), you can make $s_V$ a universal constant, though definitely not $1$ as you require. For related work, consult work by Hirsch, Itsykson and their students, for example this technical report.

Edit: As Squark comments, the runtime of Levin's algorithm also depends on the runtime of $A$, since it has to verify its answers. To get a constant $s_V$, all you need to do is to set the speeds of the various algorithms in geometric progression (rather than arithmetical progression, like in Levin's original algorithm).