Is there any documentation that covers best practices around DTM tags, especially those that cover marketing. Would like to know how we can evaluate the pixel tags on our site and what steps can be done to improve page performance.
I'm not sure what exactly your asking for. Each of the Adobe solutions provide a tool inside DTM that is used to optimize and send marketing Pixels. If your looking for a help optimizing third-party tags, you'd want to look for an optimization tool. DTM is a deployment tool rather than an optimization tool.
Something like Observe point might be better at auditing your tags.
one solution that might work is to leverage the browser's performance.getEntries function. This will allow you access all the network calls that are being called by the browser - Adobe DTM being one of them & Adobe Analytics S_Code being another one.
in these details you will see speed and file size information so you can evaluate how long it takes a specific resource to load (called duration) and how long into the total loading process it took this one resource to complete (called responseEnd)
in Analytics we built a solution that specifically capture how long it takes to for the Adobe DTM file and the Adobe Analytic s_code file to load so we can inform the IT group of how fast Adobe actually works
As long as the resource you're looking for shows up in this list, you can do the same thing with a little bit of custom java script inside of DTM.
One thing - Safari 10 or earlier do not support this feature. But starting with Safari 11 being released in Nov 2017, all major browser will support the getEntries functionality.
Unfortunately, I can't speak to "how to improve their performance"