Figuring out all those variables on your system is an important part of learning your system and then being able to adjust recipes accordingly. How much heat is lost in each step, how much volume is lost, how long it takes, overall efficiency, etc. Those are unique to your system and methods. So measure carefully and then you can take them into account the next time you brew. Some variables will differ from batch to batch, so try to learn the range of them on your system.
If you're losing too much heat from your mash tun, insulate it. Easiest way is to buy some of that duct insulating stuff with the small bubbles and aluminum kind of stuff on both sides and wrap that around your pot. Buy it at a home center. Use a bungee cord or strap or something and wrap it 2 or 3 times around. That should reduce your heat loss. Covering the pot is important, but I'm guessing you're already doing that.
Losing 4-5F over 90 min is pretty good, IMHO. I'd lose 8-10F in that same time on my half barrel system over the same time, but I get that down to 1-2F by insulating. 90 min is a long time to mash, though. Sure you need all that time?