Board updated to latest phpbb version

[Dalai]

The forum has been lagging a lot over the past few days

Not seen from here.

Neither from here.

[Fla$her]

Hmm. I have the same situation in all browsers, including when using online VPN services.
There are no problems with ghisler.com and the ghisler.ch root. Only with https://www.ghisler.ch/board/. The page keeps loading; I can see the avatars, but the content of the buttons is not visible.
Pages sometimes come to life, but more often they freeze, just like sending messages.

[white]

There are no problems with ghisler.com and the ghisler.ch root. Only with https://www.ghisler.ch/board/. The page keeps loading; I can see the avatars, but the content of the buttons is not visible.

Perhaps you can use Inspect Element, select the Network tab and look what takes a long time to load.

[ghisler(Author)]

No problem here either, and the load monitor loadavg doesn't show any high loads for the last few days.
We had some problems with Chinese web crawlers a few weeks ago, but I blocked their IP ranges.