I'm using image transforms to create 1x versions of hdpi 2x images but one area I get slammed on web page performance is loading these images. Is it possible to compress them and if so, how?
All common image file formats are compressed, even lossless formats like RAW, TIFF or PNG use compression!
To reduce file size of you images you should choose a lossy format (avoid TIFF Baseline or BMP), and accept some degree of information loss to reduce the file size. You usually set this quality to file size ratio in percentages (for the web). Craft's default quality ratio applied on transforms,
defaultImageQuality, is set to 75%, but you can always set it to your needs from the Control Panel or in your templates.
There's a nice chart on Wikimedia, illustrating the relative quality of quality settings for JPG formats.
Additionally I'd recommend you to have a look at this other question about transformation performance with image libraries supported by Craft.
Yes, you can do this if you use the Imager plugin. Here's how: Creating Optimized Images in Craft CMS
Compress? No. But you can set a default image quality for transformed images with the
defaultImageQuality config setting that can be overridden on a per-transform basis from the control panel.
Double check that the page speed reports are not flagging the images because you're rendering them at a smaller size than their natural pixel size. I believe it give a very similar message if you do.
For example; check you're not using a 1000px wide image and displaying it at only 700px. You'll want to look into the Picture element if that's happening and you're not already using it. In fact you'd want to use the picture element for different pixel densities too.