I've been playing a game on insane map size with abundant planets and about 90ish AI factions.

I currently have over 100 planets and have noticed that my turns are taking about 15-20 minutes each to do even with me going as fast as I can.

Here are the main problems that I see:

Starbase request constructor option is broken if you obsolete the base constructor that it tries to build which makes you manually build them and send them to each base that needs them, causing more time to be wasted.

Late game forming fleets is really tedious as you have to cycle through hundreds of units scattered through your huge territory which burns a lot of time and isn't fun.

For starbases the fix would be to have the "request constructor" option to build your latest constructor design, rather than the base design that has a lot of useless components like sensors and life support.

For fleet deployment, I would recommend something more drastic.  Here's what I propose:

Each ship that is finished building on any turn is put into a "queue" rather than automatically appearing at the space dock that builds it.

You can then deploy any number or all built ships in this queue at any number of space docks or adjacent territory of the space dock of yours inside your territory.  This idea is similar to the deployment system in Panzer General series if anyone remembers.  However the restriction will have to be that the ships in queue must be all deployed before turn ends.  This is to prevent people exploiting the system by stocking up on a massive fleet for burst offensives.

This allows you to rally new forces quickly without having them all scattered through a potentially massive territory in insane sized maps and you having to move them one at a time to the spot you want.  I understand there's a fleet rally point option but that isn't very viable since sometimes you don't want to deploy in the same spot and you have to change it every time you want to deploy somewhere else on each of your shipyards and late game you can have easily more than 100+ shipyards on insane maps. 

Having a build queue and freedom to deploy all your new ships to any shipyard of your choosing inside your own territory would allow for concentration of forces and bigger and more fun space battles as opposed to the scattered skirmishes over the span of massive swaths of territory that we have now.  Of course the AI should also be able to do this.  This would also save a lot of computing resources for the AI since it won't need to calculate pathing for all the ships it has built to rally to their own fleets on an insane map and result in a performance increase. 

This would also result in more epic massive multi-fleet battles, which is far more fun than the scattered skirmishes that happens on insane maps late game because there's so much territory to cover.