I was having this issue as well, to add extra info, when in street_test alone with ai disabled, at least on my machine deadlock is using about 2500mb of RAM. At around the 15 minute mark and moving around most if not all of the map RAM usage goes up to about 2900mb. I start the match with about 190-200 frames and slowly over the 15 minute period my frames dropped to about 110, and like OP said, when triggering the enemy's base/safe zone defenses frames drop significantly, in my case down below 60fps and seem to permanently stay below 100fps after doing that regardless of where you are on the map. It seems like going away from the base raises the fps from 60 implying its definitely something in the enemy base that causes the frame drops.
My first hunch was that its a memory leak but the increase from 2500mb to 2900mb is not at all worrisome as far as I know, unless there is some restriction of Source 2 that makes it a problem. CPU usage didn't seem to change much. Neither did hard drive access (not sure if the task manager HDD activity will pick up RAM paging if this is a result of paging due to game RAM limitations). I did not monitor GPU usage, I'll see if there is anything notable there and edit this post later if so.
Also to clarify this again was tested with a single hero on the map (Ivy), with the testing tools activated, and the var ai_setenabled set to 0 to stop towers/minions/creeps from attacking/moving.
