An idle hledger-ui with --watch, left running for days, may show gradually increasing CPU usage - 1%, 2%, 3%, 4%... - and RAM usage.
I have always seen this on mac, and @the-solipsist sees it on ubuntu gnu/linux. I believe it is more apparent when you have a lot of accounts / transactions. I seem to remember, or I have assumed, that it's a problem with the underlying C file-watching library (which might be different on each platform). As a workaround, you can occasionally quit and restart hledger-ui (q, C-p, enter), or suspend and resume it when not in use (C-z, fg, enter). It's quite unfortunate.

See also

• hledger-ui --watch not working on GNU/Linux ? #836
• hledger-ui --watch does not work with certain editors (gedit, IDEA) #1617
• Memory leak on OS X haskell-fswatch/hfsnotify#35
• https://github.com/haskell-fswatch/hfsnotify/issues (file watcher and windows fs interface)
• https://github.com/haskell-fswatch/hinotify/issues (linux fs interface)
• https://github.com/luite/hfsevents/issues (mac fs interface)