Sure, maybe on some NAS versions, and historically there may have been various limitations. My Synology (Intel-based DS 423+) indexes 2TB of media from scratch in 15 mins (which is fine - it's only done once), and if I throw say, 5 CD rips at it at once, by the time I've loaded my app for my streamer on my phone, the data is there and ready to go over DLNA... And Media Server (or Indexing Service in fact) absolutely does monitor changes in watched folders and it responds quickly - there nothing special to set up. I think you must have some older unit!
Also, I regularly look at resource usage on my unit (because I'm Mr. Sad) and no more than 10% of RAM memory is ever in use, indexing or not - not dissimilar to a PC perhaps....
Please us the "memory limit" on your Synology NAS - there may upgrades that will help...
In my system, there is no evidence that shows Minim (I have tried it) to have any noticeable effect on a "speed" experience -minin is more of a pain to use of course - Media Server just requires two or three clicks to set up and start it. OK, I'm not arguing about any features of minim I don't use of course!
I used to make NAS/RAID units, and know you can make a better faster units than a Synology from any old PC - but I am Mr. Expedient and Mr. Cheap, and I have no more spare PCs lying around (I gave them all away); the main cost these days is disk storage!