The delay is in downloading the first discoveries information most probably. It's database optimisation they need to make. Either that or they can try pre downloading the info in the background during the hyperjump. This requires more traffic because it will load the data even if you won't use the system map so it's traffic on fd servers for something you'll never see.
The only logical way is to reconsider how the system and first discovery data is stored and accessed. They can of course buy bigger and faster servers, which probably isn't as simple as it sounds because they would have done it already.
There's a pretty cool presentation FDEV gave talking about how the whole thing is put together, with the various databases etc. that are running on AWS (you can find it on youtube). The database records for star systems (appear to) store everything about that system, including first-discovered-by data, so it should fetch everything all at once from a single record. If they're not pre-loading it to conserve bandwidth, then there needs to be a heuristic that starts doing that once the game realizes you frequently access the system map.
I also suspected at first that the growing number of scans/discoveries are what's slowing it down, but I'm not so sure. It makes sense, since the general complaint is that it's been growing gradually slower over time for people. But really it could be anything.