Some timings for http://mathematica.stackexchange.com/q/33223/34

Timings

In[1]:= n = 400;

In[2]:= TimeAv[Plus@@ReplaceList[Product[y[j], {j, 1, n}], y[i_] rest_ :> x[i] rest];]

During evaluation of In[2]:= Total wall time is 0.554032, 
total cpu time is 0.530403 and total time spent evaluating the expression is 0.53040

During evaluation of In[2]:= The expression was evaluated 6 times, without any blocking of the runs. 
This yields a mean timing of 0.088401 with a standard deviation of 0.0074.

Out[2]= {0.088401, Null}

In[3]:= TimeAv[Sum[x[i] Product[y[j], {j, Delete[Range[n], i]}], {i, 1, n}];]

During evaluation of In[3]:= Total wall time is 0.652037, total cpu time is 0.639604 
and total time spent evaluating the expression is 0.60840

During evaluation of In[3]:= The expression was evaluated 2 times, without any blocking of the runs. 
This yields a mean timing of 0.304202 with a standard deviation of 0.14820.

Out[3]= {0.304202, Null}

In[4]:= TimeAv[Tr[Times@@@SparseArray[{{i_, i_} -> x@i, {i_, j_} -> y@j}, {n, n}]];]

During evaluation of In[4]:= Total wall time is 4.654266, total cpu time is 4.508429 
and total time spent evaluating the expression is 4.44603

During evaluation of In[4]:= The expression was evaluated 10 times, without any blocking of the runs.
This yields a mean timing of 0.444603 with a standard deviation of 0.0290.

Out[4]= {0.444603, Null}

In[5]:= TimeAv[f = Array[x, n]; g = Array[y, n]; 
 Total[f[[#]] (Times @@ g)/g[[#]] & /@ Range[n]];]

During evaluation of In[5]:= Total wall time is 0.565032, total cpu time is 0.577204 
and total time spent evaluating the expression is 0.57720

During evaluation of In[5]:= The expression was evaluated 3 times, without any blocking of the runs. 
This yields a mean timing of 0.192401 with a standard deviation of 0.0074.

Out[5]= {0.192401, Null}