The problem with opening sites - where to dig?

Question

linux

Recently, some sites have shown such a hat.
You go to the site, the title of the page appears - and the icon on the tab spins and spins. The page is empty.
I try another browser - the same.
I try to download this page with wget - it downloads 28 kilobytes and idles the cursor.
Site examples:
alv.me/ocz-revo-drive-xronika-neudach-seriya-1/
vladprombank.ru/currency-rates/ The
symptoms are exactly the same (the first one is on WP, the second one is Bitrix).
The site does not want to load for a long time, then it either fizzles out, or finally loads. From the second or third time - it loads and works as if nothing had happened.
Tor opens both sites instantly. It turns out, some kind of crap from the provider? Or me?
There are two providers - MTS and local - the picture is the same.
On an Ubuntu machine, the Internet distributes Mikrotik configured for these two providers (local as a backup - while MTS is running, it is not used).

Answer 1

/ip firewall mangle
chain=forward action=change-mss new-mss=1452 tcp-flags=syn,!rst in-interface=all-ppp protocol=tcp tcp-mss=1453-65535
chain=forward action=change-mss new-mss=1452 tcp-flags=syn,!rst out-interface=all-ppp protocol=tcp tcp-mss=1453-65535
Try it. Choosing the size of packages.