mail/docs/performance.md

Memory use

This page explains how the memory figures in the README and on flectar.com/mail are measured, and how to check them yourself.

Results

A fresh launch with one IMAP account (860 messages), with the inbox open and an HTML newsletter displayed, sampled 60 seconds after launch (one Thunderbird run at 100 seconds). Both apps ran on the same Linux machine against the same mailbox, in a 1320 × 800 window. Each range covers two runs.

Memory (MiB)	Flectar Mail	Thunderbird 159
Private memory	19.0–19.1	239.7–242.3
PSS	80.9–81.3	414.3–417.4
RSS	87.2–87.3	556.3–560.4
Processes	1	4

Thunderbird's RSS is the sum over its processes. Its first launch, which also downloaded the mailbox, measured 293.8 MiB private and 479.4 MiB PSS.

The same Flectar Mail build in other states:

Flectar Mail (MiB)	Private	PSS	RSS
Welcome screen, no account	8.2	55.1	61.1
Inbox idle after the first sync	20.2	79.9	86.6
After reading two messages	25.8	93.0	100.0
Then minimized for 25 seconds	24.4	91.8	98.9

The README rounds these to megabytes (1 MiB = 1.049 MB). It quotes about 20 MB of private memory and about 85 MB PSS for Flectar Mail, and about 250 MB and 435 MB for Thunderbird.

What the numbers mean

Metric	Counts	Where you see it
Private memory	Memory that belongs to the app alone and isn't backed by a file: heap, caches, stacks. Adds up correctly across processes.	Windows Task Manager's Memory column (private working set); RssAnon on Linux
PSS	Private memory plus the app's code and libraries mapped from disk, with shared pages split between the processes that use them	smem, /proc/<pid>/smaps_rollup
RSS	Every resident page, shared libraries in full. Summed over several processes, it counts shared pages more than once.	top, htop (RES)

Most of Flectar Mail's RSS is its own executable: about 59 MiB of code and read-only data mapped from disk. Those pages are clean, so the system can drop them under memory pressure and read them back later.

How to measure

Keep everything but the app identical: machine, account, mailbox, window size and renderer. Use the same state every time: a fresh launch, the inbox open, a message displayed and no interaction for 60 seconds. Repeat each measurement at least twice.

Linux

scripts/generate-benchmark-mailbox.py ~/bench/Maildir   # test mailbox; serve it over IMAP
scripts/memory-snapshot.py "$(pidof flectar-mail)"
scripts/memory-snapshot.py --exe-dir /opt/thunderbird   # every process of an app

Windows

In Task Manager, open Details, right-click a column header and choose Select columns. Add Memory (private working set), Working set (memory) and Commit size. For an app with several processes, add up all of them; the Processes tab groups them for you.

Keep the window open while you read the numbers. After Flectar Mail has been minimized for 10 seconds, or closed to the tray, it releases its rendering buffers and trims its working set, so Task Manager then shows the background figure. Commit size counts all the private memory the app has committed, whether or not it is in RAM at that moment, so it doesn't drop after a trim.

macOS

Activity Monitor's Memory column shows the physical footprint, which also counts window surfaces held for the display, so it reads higher on Retina screens. footprint <pid> or vmmap --summary <pid> gives the breakdown.

Test setup

• Flectar Mail: main at 2cdc2a2 (2 November 2026), the CI AppImage, with the default CPU renderer. The optional GPU renderer uses more memory and isn't covered here.
• Thunderbird: 159.0, Mozilla's Linux x86-64 build, a new profile with default settings.
• System: Debian 13, Linux 6.12, x86-64. Xvfb at 1920 × 1080 with Openbox, and both windows at 1320 × 800 and scale 1.
• Mailbox: Dovecot 2.4.1 on the same machine, serving the Maildir written by scripts/generate-benchmark-mailbox.py. It has 600 Inbox, 200 Archive and 60 Sent messages. About 45% are HTML newsletters, 20% short HTML notifications and 35% plain-text conversations, some with PDF attachments.
• Connections: Flectar Mail used IMAP over TLS; Thunderbird used plain IMAP. Thunderbird lists only subscribed folders, Inbox and Trash here, so it synced fewer folders than Flectar Mail.