How to get better performances on my forum?

I've already sent a support ticket, but he (Lee) told me to give a look at the community, because he said to me "on our end looks to be functioning normally".

My problem is that since (about!) 14:00 of today, my forum is working sluggishly. I don't understand why … I've not changed the configurations or added new softwares, daemons and so on. Nothing.

Usually (it happens only today) it becomes slower when I try to reply/read a single topic, but it's not a rule. When I try to load the unread posts it's fast, while it should be slower - since it's a more complex query. I don't know what the problem could be. :?

This is the MTR output:

user@partenopeo:~$ mtr 151.75.29.222 --report
HOST: partenopeo Loss% Snt Last Avg Best Wrst StDev
1\. 212.111.33.229 0.0% 10 0.5 0.5 0.4 0.7 0.1
2\. 212.111.33.233 0.0% 10 0.5 0.4 0.4 0.5 0.0
3\. te3-1-border76-01.lon2.telec 70.0% 10 0.8 0.8 0.8 0.8 0.0
4\. 85.90.238.45 60.0% 10 1.1 1.1 1.0 1.2 0.1
5\. 217.20.44.194 30.0% 10 0.9 0.9 0.8 0.9 0.0
6\. linx.wind.it 0.0% 10 26.0 26.0 25.9 26.1 0.1
7\. 151.6.0.157 0.0% 10 25.7 30.2 25.6 71.5 14.5
8\. 151.6.1.86 0.0% 10 42.4 52.7 42.4 101.8 19.3
9\. 151.6.4.102 0.0% 10 46.0 45.9 45.9 46.0 0.0
10\. 151.6.104.86 0.0% 10 46.1 46.2 46.0 47.0 0.3
11\. 151.6.104.137 60.0% 10 40.7 41.3 40.7 42.6 0.8
12\. 151.75.29.222 0.0% 10 53.5 52.9 51.7 54.8 1.0

marco@home-net:~$ mtr partenopeo.net --report
HOST: home-net Loss% Snt Last Avg Best Wrst StDev
1.|-- 192.168.1.1 0.0% 10 0.6 0.6 0.6 0.7 0.0
2.|-- 151.6.154.72 90.0% 10 13.8 13.8 13.8 13.8 0.0
3.|-- 151.6.104.134 0.0% 10 26.3 22.3 14.6 39.1 7.7
4.|-- ??? 100.0 10 0.0 0.0 0.0 0.0 0.0
5.|-- 151.6.4.101 0.0% 10 32.8 68.6 27.5 208.3 57.5
6.|-- 151.6.1.218 0.0% 10 16.7 28.9 16.7 66.5 14.8
7.|-- peer1.ams2.telecityredbus 0.0% 10 53.9 60.7 53.2 73.6 6.8
8.|-- 85.90.238.45 80.0% 10 55.6 54.0 52.4 55.6 2.2
9.|-- te4-1-dist65-01.lon10.tel 0.0% 10 54.4 55.3 52.5 61.0 2.9
10.|-- 212.111.33.234 0.0% 10 55.9 58.4 55.3 70.5 4.6
11.|-- partenopeo.net 0.0% 10 52.3 58.1 51.6 70.4 6.3 

while this is the free -m output:

user@partenopeo:~$ free -m
total used free shared buffers cached
Mem: 497 449 47 0 8 285
-/+ buffers/cache: 155 341
Swap: 511 87 424 

Btw: he told me about an OOM problem, but as I've stated: I've not changed anything. The number of connected users is always the same (a range between ~70 and ~150).

<4>lowmem_reserve[]: 0 0 0 0<4>DMA: 231*4kB 277*8kB 50*16kB 3*32kB 0*64kB 0*128kB
0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB 

Please, can you help me?