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except that here I do not see what could cause that 
CPU is not fully used
RAM is used at 1/4th
There aren't queries waiting and piling up like when it was becoming slow in the old server...
Maybe it is just OVH's network which is a little full
You can see at the bottom of the page the time it took to the server to generate it. Usually it is below 0.5 second. If you notice on some particular page that the time is always higher than that, you may report these pages here. But for example right now it took few seconds to load a page but it was generated in 0.008 sec: it is a network problem and not a server problem (luckily :D)
It may also sometimes be an attack made to the server, I had that last weekend: for one hour it was very slow to respond, because another server was "attacking" it
CPU is not fully used
RAM is used at 1/4th
There aren't queries waiting and piling up like when it was becoming slow in the old server...
Maybe it is just OVH's network which is a little full
You can see at the bottom of the page the time it took to the server to generate it. Usually it is below 0.5 second. If you notice on some particular page that the time is always higher than that, you may report these pages here. But for example right now it took few seconds to load a page but it was generated in 0.008 sec: it is a network problem and not a server problem (luckily :D)
It may also sometimes be an attack made to the server, I had that last weekend: for one hour it was very slow to respond, because another server was "attacking" it