On my laptop, with Intel® Core™ i5-5200U CPU @ 2.20GHz (2 cores, 4 threads) and a GeForce 940M (384 cores), I get:
GPU
1024x1024: 8’51"
512x512: 7’28"
256x256: 6’55"
128x128: 7’05"
64x64: 8’37"
32x32: 15’5"CPU
1024x1024: 27’12"
512x512: 16’11"
256x256: 10’50"
128x128: 9’54"
64x64: 9’41" (hilbert: 9’48")
32x32: 9’43" (hilbert: 9’33")
16x16: 9’41" (hilbert: 9’28")
8x8: 9’44" (hilbert: 9’21")
So, the GPU does save me about 30% at best, 256x256 seems the better configuration in my case. Big difference with my PC at work with its Intel® Core™ i7-5960X CPU @ 3.00GHz 3.00 GHz (8 cores, 16 threads) and its GeForce GTX TITAN X (3072 cores), where I get:
GPU
1024x1024 49"
512x512 48"
256x256 59"
128x128 1’30"
64x64 3’31"
32x32 10’23"
16x16 32’52"960x540 49"
480x270 50"
240x135 1’4"
120x68 2’6"
60x34 5’50"CPU
1024x1024 19’10"
512x512 11’10"
256x256 3’20"
128x128 2’05"
64x64 1’53"
32x32 1’52"
16x16 1’51"
8x8 1’52"960x540 20’2"
480x270 5’19"
240x135 2’28"
120x68 2’3"
60x34 1’51"
30x17 1’51"
15x9 1’53"
So here, I can go as large as I want (at least for 50% of 1080p resolution), I think I did the test where, for 100%, I saw that going over 1024x1024 did not improve anymore …