The Dota 2 community has been complaining about performance for as long as the Source 2 version of the game, Reborn, has been in the main. It seems every patch reduces performance for someone, somewhere, somehow. In one of many threads complaining about this, and asking for a spring cleaning update to help them, Valve posted a link to their performance testing instructions – it should help people identify problems.
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Their thread for this is over on the official dev forums. The instructions are relatively simple, involving enabling the dev console and then running a short process that plays with your graphics settings and records frame data, as well as analysing your machine for possible problems.
Having ran it as a test just now, it takes no more than a minute or so, and gives plenty of easily-readable output in the console. As noted by Drew Bliss there, condump can be used to turn this into a text file to more easily share.
Bliss mentions a large number of things that can help, such as turning off Windows 10 Game DVR and trying various settings. This probably isn’t Valve’s last word on the subject of Dota 2 performance, but hopefully does lead to them getting the data they need to fix things.