This is really good news, I'd been hoping to see a performance-based release for some time as traditionally IPS does rate low in Page SpeedGTMetrix/Yslow tests and we've been waiting for some of these overdue improvements for quite some time. So huge thanks!
Lazyloading should really help with Gallery in particular, I currently get a poor Pagespeed score of F42 on one of my sites.
Please consider adding to your to do list (if not already added):
Serve scaled images (current GTMetrix reports a score of F0 with a 1.2MiB 80% possible saving is possible on my Gallery index page)
Where static content is uploaded to Amazon S3 and Cloudfront (it should be copied, not moved!), often there is missing header metadata such as cache-control, expires etc. Whilst investigating poor Cloudfront cache hit ratios of just 5%, I noticed this week an IPS uploaded .png image placed in my S3 bucket with a content-type of image/jpeg instead of image/png.
Leverage Browser Caching (you can't do anything about Amazon and Google external resources, but IPS could and should add more to improve caching of its static resources)
Specify Image Dimensions - I get awful scores for this both on Gallery and Forums where images are missing width and height attributes. This happens particularly in grid view layout and where you upload an image attachment into forum description fields via AdminCP.
Remove query strings from urls - Have raised this several times, and keep getting told it's needed to allow cache busting, but the string token could be built into the actual filename/url itself. If CSS for example is later rebuilt, a new url is created and immediately browsers see the updated resource and recache it.
All of the separate CSS and JS files should be grouped into one or two files, not like 10-12.
Pages article images should have medium sized thumbnails available instead of just tiny thumbnails which look awful stretch to cover on 4K displays. The alternative is full-sized (in my case massive 1920x1080 images) which kill page loading times are are such overkill for article listing previews.
