Mortar does not start: it shows why, with a button that opens the folder holding mortar.log. Starting Mortar again while a copy is stuck offers to end it.
Run checks: Settings › About › Diagnostics › Run checks…; Repair fixes what it can.
Logs: mortar.log in the data folder, plus crash.log after a crash. Each profile's SMAPI log is on its log view; Share log… there uploads it to smapi.io/log and gives you a link.
Slow start: mortar.log times each step of the start (lines beginning startup:). For more, the Mortar menu's Restart and record startup profile records Mortar's first 30 seconds into the data folder's startup-profile folder, and the next diagnostics zip includes it.
Slow scrolling or loud fans (Linux): Settings › Appearance › Display › Graphics set to Smoother scrolling keeps the graphics card drawing the window, which some NVIDIA setups otherwise hand to the processor; Compatible is for a window that stays blank or white. Either applies after a restart, and mortar.log names the one in use (its msg=graphics line). A window too blank to reach Settings: start Mortar once with WEBKIT_DISABLE_DMABUF_RENDERER=1 in its environment, which overrides the setting, and pick Compatible.
Report a bug: the Mortar menu's Report a bug opens a prefilled GitHub issue. Leave Attach diagnostics ticked to save a zip (checks, logs, settings and recent runs, with secrets and your home folder removed) and drag it into the issue. Save diagnostics… (Mortar menu, Settings › About › Diagnostics) writes the same zip. Report a bug can also copy the report for a new post in the bug-reports forum on Mortar's Discord.
Ask for help or an idea: the Mortar menu has Ask on Discord and Suggest a feature….