With this customer ticket, the analysis phase will be an important activity. The main question off course is why is the website slow? And when did the website started to slow down? Was it after installing a NEW plugin e.g.? I would ask feedback around the latter question.
I would check the following:
– are there (lots of) images with a size that is just too big?
Unless one want to deliver HQ images for download, images > 1MB are too big. Resizing them to reasonable formats (dimension, compression) is key here.
Ideally people upload their media in already a not so big format (makes backups also less big). This can be enforced (php.ini and/or htaccess).
With existing media, luckily there are some image reduce plugins, like EWWW Image Optimizer.
– is the initial connection taking too long
In that case, it might be a hosting problem: capacity (too much traffic at that server), limited resources (memory, CPU).
– are there lots of HTTP requests?
If we see there are e.g. more than 100 HTTP requests, chances are there are way too many plugins or too many media requests. If it seems there are too many plugins, we should investigate if there are plugins that aren’t need any longer.
Solutions that should be considered as well (depending on the root cause).
Using a CDN (e.g. Cloudflare) or a caching plugin so specific files (e.g. images) will not always be served by the hosting server but by a (local) CDN server. Enable compression (using a plugin or directly into .htaccess or alike depending on webserver).