In some ways it's a benefit, one of the reason MPLAB X is so slow is that it parses all your open projects on start up whether you want it to or not.
I don't find it slow. It starts and it's ready parsing the project in 12 seconds. For me, that's acceptable for a program that I start
in the morning and leave open the whole day.
Btw, why should one have to open multiple projects at a time?
Btw2, I don't use MPLABX to actually write code. For that I use Kate, one of the best code editors ever.
When I save something in Kate, MPLABX automatically get notified and updates the content in the IDE.