I thought I would try some speed tests and the results are below; the images were ones I found on my hard drive - nothing special about them. There are only one lot of results for each image below but I ran the tests a few times and there was not a great difference in the results.

Using a 0.8MB jpg image

Method	Time seconds	File size	Width x height
Original image		853789	1667 x 2500
Using Imagick	0.431061983109	7052	100 x 149
Imagemagick thumbnail	0.573105096817	7074	100 x 150
Imagemagick thumbnail and the "size hint"	0.293253183365	6951	100 x 150
Imagemagick resize	1.31543397903	15992	100 x 150
GD	2.12855386734	5416	100 x 149

Note: GD failed with a memory error at any images over this size. I assume I could allocate more memory but it shows ImageMagick uses less memory ?

Using a 2.8MB jpg image

Method	Time seconds	File size	Width x height
Original image		2962005	1667 x 2500
Using Imagick	0.569258928299	20539	100 x 149
Imagemagick thumbnail	0.713599920273	20602	100 x 150
Imagemagick thumbnail and the "size hint"	0.204536914825	20260	100 x 150
Imagemagick resize	1.46319389343	29484	100 x 150
GD	2.22840690613	4986	100 x 149

Using a 1.26MB png image

Method	Time seconds	File size	Width x height
Original image		1330464	647 x 476
Using Imagick	0.199882030487	15889	100 x 73
Imagemagick thumbnail	0.244040966034	38564	100 x 74
Imagemagick thumbnail and the "size hint"	0.225668907166	38564	100 x 74
Imagemagick resize	0.232110023499	38187	100 x 74
GD	0.420181035995	13157	100 x 73

Note: For some reason GD did not retain the transparency