Are there huge time savings

In this case, absolutely none. Both forms are compiled down into constant strings:

• Double quoted:
  

  % perl -MO=Concise -e 'print qq{hello, world}'
  6  <@> leave[1 ref] vKP/REFC ->(end)
  1     <0> enter ->2
  2     <;> nextstate(main 1 -e:1) v ->3
  5     <@> print vK ->6
  3        <0> pushmark s ->4
  4        <$> const(PV "hello, world") s ->5
  [download]

• versus single quoted:
  

  % perl -MO=Concise -e 'print q{hello, world}'
  6  <@> leave[1 ref] vKP/REFC ->(end)
  1     <0> enter ->2
  2     <;> nextstate(main 1 -e:1) v ->3
  5     <@> print vK ->6
  3        <0> pushmark s ->4
  4        <$> const(PV "hello, world") s ->5
  -e syntax OK
  [download]

The compiler will also fold things like "hello, world\n" and 'hello, world' . "\n" into the same constant strings. It's only when interpolation in a double string comes into play that things change:

% perl -MO=Concise -e 'print qq{hello, $world\n}'
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
9     <@> print vK ->a
3        <0> pushmark s ->4
-        <1> ex-stringify sK/1 ->9
-           <0> ex-pushmark s ->4
8           <2> concat[t2] sKS/2 ->9
6              <2> concat[t1] sK/2 ->7
4                 <$> const(PV "hello, ") s ->5
-                 <1> ex-rv2sv sK/1 ->6
5                    <$> gvsv(*world) s ->6
7              <$> const(PV "\n") s ->8
-e syntax OK
[download]

... which is only slightly different to...

% perl -MO=Concise -e 'print qq{hello, } . $world . qq{\n}'
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
9     <@> print vK ->a
3        <0> pushmark s ->4
8        <2> concat[t4] sKS/2 ->9
6           <2> concat[t2] sK/2 ->7
4              <$> const(PV "hello, ") s ->5
-              <1> ex-rv2sv sK/1 ->6
5                 <$> gvsv(*world) s ->6
7           <$> const(PV "\n") s ->8
-e syntax OK
[download]

The above both amount to much the same thing. So basically, you can always use double quotes, interpolate when you need to, and do not worry about performance.