I'll assume your list of 2D examples, this can easily be extended to arbitrary dimensions, and for that matter to a list with elements of differing depths. On a quick test using

Table[RandomInteger[100, {RandomInteger[{50, 100}], RandomInteger[{50, 100}]}], {2000}];

to generate 2000 randomly sized 2D arrays, over 30X faster than reco:

ranger[list_, lens_] := With[{x = Accumulate@lens},
  Inner[list[[# ;; #2]] &, Most@Prepend[x, 0] + 1, x, List]]

Use example:

test = {RandomReal[10, {2, 3}], RandomReal[20, {5, 5}]};

dims = {{3, 3, 5, 5, 5, 5, 5}, {2, 5}};

target = Flatten[test];

Fold[ranger, target, dims] == test

(* True *)

Note the "dimensions" argument is specified from the "bottom" up, per construction element. This allows, among other things, the targets themselves to be ragged.

I'll assume your list of 2D examples, this can easily be extended to arbitrary dimensions, and for that matter to a list with elements of differing depths. On a quick test using

Table[RandomInteger[100, {RandomInteger[{50, 100}], RandomInteger[{50, 100}]}], {2000}];

to generate 2000 randomly sized 2D arrays, over 30X faster than reco:

ranger[list_, lens_] := With[{x = Accumulate@lens},
  Inner[list[[# ;; #2]] &, Most@Prepend[x, 0] + 1, x, List]]

Use example:

test = {RandomReal[10, {2, 3}], RandomReal[20, {5, 5}]};

dims = {{3, 3, 5, 5, 5, 5, 5}, {2, 5}};

target = Flatten[test];

Fold[ranger, target, dims] == test

(* True *)

Note the "dimensions" argument is specified from the "bottom" up, per construction element. This allows, among other things, the targets themselves to be ragged.

Btw- ranger is simply a ragged partitioner I built long ago: given a flat list and a list of lengths, it returns the original list partitioned by the lengths. IIRC, faster than the (undocumented) built-in.