The most meaningful measure of database size is not the disk storage size, but the total number of words in the database. What’s your total word count displayed in File > Database Properties?
A database holding plain text files could easily be a thousand times larger in total word count than a database holding PDF files, although both have the same file size.
I can run my standard set of five databases, which have a total of 6.2 GB file size, on my MacBook Air with 4 GB RAM, at full speed, without slowdowns (as long as there remains several hundred MB of free physical RAM). But I would expect very poor performance if that 6.2 GB of database files contained only plain text document files. My set of open databases contains just under 40 million total words. I expect most of my searches to take 50 milliseconds or less, Classtfy and See Also suggestions to appear in no more than a second or two, and spinning balls to never appear.
Launch Apple’s Activity Monitor app, which allows checking the amount of free physical RAM, the number of page outs occurring and the size of Virtual Memory swap files. When your Mac runs out of free RAM, Virtual memory takes over, allowing continued processing of data by swapping data Bach and forth between RAM and disk-based swap files. But heavy use of swap files results in slowdowns, sometimes long pauses during which the dreaded spinning ball appears.
My guess is that the slow performance you see is happening because the amount of free physical RAM is so low that Virtual Memory is busily moving data back and forth between RAM and swap files. Between the memory needs of your database, the memory demands of other applications and the accumulation of “crud” inactive data stuck in RAM, free RAM may vanish in time. That’s not good!
Depending on the total word count in your database, you might consider splitting your database into or more databases so that they can individually run without slowdown, along with other applications you wish to keep open. I’ve been doing that for years. Altogether I manage more than 250,000 documents among a number of databases, each of which meets a particular need or interest. Needless to say, I couldn’t fit all of them onto my Air’s 256 GB SSD. That’s fine; I spend about 99% of my time working in the set of databases that I keep on the Air. If needed, I could access rarely used databases from an external drive mounted on the Air via Thunderbolt. (The read/write speeds of the Air’s SSD and the Pegasus Thunderbolt RAID unit make my quad-core iMac feel bog slow.)
Apple has just released a Safari update, and the release notes say that memory management is improved. I hope so, as the previous version soaked up RAM more than any other app on my Mac, including sticking a great deal of currently inactive data in RAM. I found that, with only Safari open, I could get it to eventually use up all free RAM - even on my iMac with 8 GB RAM.
I use two utilities to avoid running out of free RAM, so as to keep my databases at top speed, Although I frequently preach about the potential hazards of installing hacks, I use one called MenuMeters that allows me to monitor memory usage in the menubar. When I see that free RAM has dropped to about 700 MB, I click on the Purge button in C0cktail to clear inactive data from RAM and optimize memory. Presto! I’ve once more got plenty of free RAM “headroom” and the Air never slows down.