Last time I administered a Filemaker SERVER was quite a while ago (Filemaker 12 was the newest version I used), but at the time you could adjust how much memory Filemaker Server used and I think how much it used for Cache on the server. By turning that up I was able to get some significant speed improvements. I keep looking for something like that in the admin console and can't find it. Is that something mortals can still control?
Since they botched the web admin tool in the filemaker 17 days, I think the lost settings are via command line fmsadmin tool only
Although there are probably newer published articles, you may find this useful from:
For cache memory size, FileMaker Server defaults to 512 MB. You can use the command-line interface or the Claris FileMaker Admin API to change the cache size based on the server's load and its available memory. The default cache size and the ability to change it aren't new in version 19.1.2, but it's worth noting that you should try to optimize the cache size. The best approach is to monitor Cache Hit % in the Stats.log file. This value is the percentage of times FileMaker Server retrieved data from cache memory rather than from the hard disk. If it's less than 90 to 95%, then you should increase the cache size until it's as close to 100% as possible