How fast graphics download has nothing to do with whether they are customized or not, only with the file size. When preparing images for web pages you have to balance file size with clarity.
The JPEG file format allows for a range of compression of images to achieve an appropriate file size. By file size we mean not the size of the image in pixels across and down, but the size of the file in storage or on disc.
A more highly compressed image will tend to look fuzzy or blocky, but the size of the file will be small. A minimally compressed image will be clearer, have clear detail, but will have a larger file size. Deciding on how much compression to apply when converting the image from the hi-res original to a web appropriate jpg is a balancing act.
Many people new to starting a web site get confused by the difference between image size and file size. Also, web browsers will shrink or expand an image to fit the size noted in the page’s code. So someone may upload a high resolution image appropriate for printing to the server and when they view the web page it may look small, if the code has a particular width and height noted. But the full-size image must be downloaded and then the client browser adjusts it to fit the space allotted. So large image files take a long time to download, even if they look small on the web page.
To make sure your custom images download quickly you must prepare them for the web, not merely upload a hi-res image and hope for the best.
Here is a detail from a 1933 woodblock print “Cat and Goldfish” by Ohara Koson with increasing compression applied to the file. You can see how as compression increases there are more and more compression artifacts, what many would call pixelation. If you look at the fine details you can see how high compression means loss of detail.
Ohara Koson - Cat and Goldfish detail - quality 100% - file size 295kB
Ohara Koson - Cat and Goldfish detail - quality 75% - file size 65kB
Ohara Koson - Cat and Goldfish detail - quality 50% - file size 47kB
Ohara Koson - Cat and Goldfish detail - quality 25% - file size 35kB
Ohara Koson - Cat and Goldfish detail -quality 5% - file size 22kB
Here’s the full print.
Ohara Koson - Cat and Goldfish 85% 165kB
I hope this brief explanation will be of some help.